/Building Dynamic Websites at Harvard – Lecture 6

Building Dynamic Websites at Harvard – Lecture 6

Video: Building Dynamic Websites at Harvard – Lecture 6


[ Silence ] >> Okay. Welcome back to Computer Science S-75. This is Lecture Six, JavaScript. This seems to be fun to start these classes by asking what we did last time because no one ever seems to know. So let's go there first. So, last time on Monday we dived a bit deeper into SQL, talked about a few topics, among which were? Axle? >> Well, joining tables. >> Okay, so joining tables which refers to what? >> It's defining something in the tables [inaudible]. >> Okay. >> Which refers to a similar field in another table of these two and [inaudible] join those two together and simply gives you the things associated with that [inaudible] from one table and puts it next to another one.

>> Okay, good. So just to summarize, if you have a relational database, inside of which is at least two tables and one of those is a primary key, like an ID, and another of those is the same ID but not in the same kind of relationship. You can join those two tables on that shared ID so that you get back essentially the union of the rows from this table with the corresponding rows from this table. And when I said it's the idea is a little bit different in the second table, we call this a foreign key, because it's there not necessarily to uniquely identify the row but to somehow link that row back to a corresponding primary key. So what was an example that we did involving primary and foreign keys? >> We had an inputting table and then we had a sales table — >> Okay. >> — so we joined the actual sales to the [inaudible]. >> Okay, good.

So we had an order — a sales table, an orders — sorry. We had products, we had orders, and there were some keys shared among those tables, for instance, an employee ID. Sorry, we had an employees table, and we had an orders table, and the shared key between those two tables was some employee identifier. So what's an upside of actually factoring out something like an order into an orders table? What was the motivation for introducing two tables on Monday as opposed to just one bigger table? Like if we already have an employees table on day one and we only have that one table, why not just add another column that lists the product ID that that person sold? Jack? >> [Inaudible] a whole lot of redundant information that's taking up space on wherever [inaudible]. >> Okay, good. You could end up with a whole bunch of redundant information.

For instance, if you clutter the employees' table with all of your orders you might have Jack, Jack, Jack, just because Jack sold three different items, and we certainly don't need three different copies of his name. And alternatively, if we didn't — if we weren't as bad as that, if we didn't actually just duplicate Jack's name or the employee's name again and again, what if we instead just put another column in the employees' table? >> Well that wouldn't be good either because that would be like faking a table because you would have to use commas or some kind of [inaudible] characters to actually identify the individual sales. >> Okay. So, okay. So, you've interpreted my proposal of another column, let's call it the orders column, as potentially containing zero or more order ID's, or product ID's, and we would have to resort to some kind of hack whereby we just separate those product ID's by commas and at that point we might as well just have multiple columns.

Okay, so let's do that. That was a stupid mistake I made the first time, let me have one column per order in the employees' table. Why not go that route? Why introduce this additional complexity of a whole other table? Let me sort of — let me play naive for a moment. So what would I name this column? I'm going to call it Order One. What would I name the second column? Order Two. What do I name the third column? Order Three. Now, start to find faults with my proposal. Jack? >> Well, so you have one customer — well, one employee that sells, let's say, 300 items. >> Okay. >> And another who sells maybe two or three. >> Okay. >> First of all, now you have 300 columns in your — >> Good. >> — table, which is way more than you need for the one who's only sold two or three items. And let's say if you [inaudible] the customer that sold 300 items but you want to keep the sales you wouldn't be able to do that because they're all on the same page.

>> Excellent. So a couple problems arise here. One, the table just gets really wide. For instance, to handle an employee who has sold as many as 300 items you now need Order One, Order Two, Order dot, dot, dot, Order 300. Which literally means 300 columns, but remember this is a spreadsheet of sorts, it has rows and columns and it has the same number of columns for every one of the rows, so now just because you have a really good salesperson he or she has 300 orders to his or her name, but suppose the norm is far fewer than that. Or you have a guy who's not doing so well and he only has two orders to his name, well you're wasting it seems a huge amount of space. Even if we've not filled in the blanks there's definitely some overhead involved in having all of these columns there for all of the rows.

So you just have this very ragged edge to your table whereby it might be 300 orders long, it might be two orders long, but this doesn't feel like the best design. As moreover, as Jack points out, if you delete or fire an employee, you're going to lose in this model all of his or her order numbers, which doesn't seem ideal. Now we can mitigate that one by instead of deleting the employee we could just have another field that's called terminated and it's one if yes or zero if no. So that might mitigate that but still this doesn't feel like the cleanest design. So the where we ended up on Monday was pretty good. So the [inaudible] schools table still had some other faults with it but at least this factoring out of orders and having it as a distinct table from employees was a good thing because we can always recover the relationships by doing joins, whether implicitly or explicitly. So what was the analog now for project one? Even though you probably haven't dived into it yet project one involves stocks, and portfolios, and users, and so forth, so what's the similarity there with this project? Axle? >> Maybe you would want to have one table containing all the portfolios on your entire site.

>> Okay. >> And another table containing all your user names and passwords. >> Okay, good. So maybe you'd have one table for all of your portfolios. And on Monday we defined a portfolio somewhat arbitrarily as being a stock symbol, a quantity, and it also had one other field associated with it, in the portfolios table. Stock symbol, quantity, and? >> Sales price? >> Could have sales price if we wanted to remember that information, but we crucially need one other field in this portfolios table. Yeah? >> How much money is [inaudible] sorry. >> Okay. Retracted. So I've got my users table, now I've got my portfolios table, and at the moment my portfolios table only has quantity and symbol. >> You need to tell — you need to [inaudible] which user actually has [inaudible]. >> Exactly.

We need the user ID. We need some kind of identification for the user so that we know who owns four shares of GOOG, in the case of a Google stock. Now we could store price if we did care about that history, and that's ultimately up to you, the [inaudible] allows you to simplify and allows a user to sale all of his or her shares, not just individual ones. So you don't really have to know exactly which of your shares you're selling. But that could be a compelling feature. Now in terms of this user ID, let's go there. In the users table we presumably have a username, which might as well be email address, password, or a hash thereof. Shouldn't be storing clear text passwords even though we did initially for John Harvard. What should the primary key be in the users table for project one? And again, don't take this as law, different designs are possible, but what could be? Yeah? >> A unique identifier. >> Okay. Unique identifier, I propose an email address.

Like it or hate it? Or do you like it or hate it? >> Well, you can do email address. >> Okay. So you say, okay, you kind of like it. Email address, who dislikes email address as the unique identifier as the primary key for my users table? Ben? >> It seems to be easier just to use a number. >> Okay. >> If you're [inaudible] who owns [inaudible] a number [inaudible] link that to an email address. >> Good. So just to recap, the upside of using a numeric identifier, like an int, that just auto increments, would be one, it's a little more efficient because you just search on a 32, or 64-bit value rather than some arbitrary length varchar. And you can also index it a bit more readily because there's just less — there's fewer bits to actually index. So it feels like that should be higher performing, certainly for large datasets. Jack? >> You're also saving space from also repeatedly putting in whatever someone's email is and however long it is when you could just be putting in some sort of — >> Exactly, so the space savings can certainly add up because if this user owns a lot of stocks and his or her email address was in every row of the portfolios table it just feels kind of ridiculous to store 10 or 12 or 20 characters just too uniquely identify that user.

Moreover there's another problem that would arise. Suppose that I wasn't this savvy, I didn't use numbers, I just used email address. Think about other corner cases that arise — could arise, not necessarily in project one, but if you continue to roll out project one as sort of an actual commercial website. What's a design flaw you might regret a few months later? >> Email addresses change, they disappear [inaudible] password. >> Good. [ Inaudible Speaker ] Good. So you leave Harvard and you lose your Harvard.edu address. You leave your company; you just decide you got to stop using that AOL.com address. You want to change your email address. Now what's the implication? Well, you the programmer could certainly allow a user to change his or her email address, but how many different tables or rows are you going to have to update now? If they change their email address you have to minimally update what? >> You have to — if you have a separate table for user ID's and emails then it's easily done you just need to deal with that table. But if you have it on all stocks it's a mess.

>> Exactly. Right? You have to do an update on the users table, you have to do an update on the portfolios table, or wherever else you've used it. And then there's the issue of well, how do you do that all atomically because it'd kind of be ideal that this kind of change happen all at once and not a little bit of change over here, then over here, then over here, because for whatever reason if that user decides to hit reload in some other browser window in which they're already logged in you could end up with some very funky undefined behavior if some of your tables have malan@harvard.edu and other tables have malan@yale.edu, just because you happen to look at your database while that change was in process. Jack? >> And beyond that if you do have some sort of [inaudible] atomic process so it does it all at once you're going to shut down the — using the SQL database for all the other users who [inaudible].

>> Well, good. So if you resort to lock specifically and you're using MyISAM tables, which was one of the formats that you can have for a table, then you would indeed have to lock out other users. In this case you could use NoDB, which now is in fact the default, and transactions would not be as — would not have the same impact on users. So we can at least mitigate that. But there's then at least the additional complexity of just updating all of these darn tables just because you made a foolish design decision upfront. So, realize that when I say there's many different ways of doing these things, it's fine if you still for this project, for instance, want to use email address as your identifier, but you're going to have to justify that to us. For instance, in the [inaudible] that you were asked for project zero and also now for project one, you're invited to explain your various design decision, so if you feel strongly about something that's fine. But what we're really looking for when evaluating projects is did you give it the thought and did you make a judgment call as opposed to just picking something because it seemed easy without really thinking through the implications? So do be mindful of those kinds of things.

So in terms of project zero, it has been distributed among the TF's. You should soon, if you haven't already, heard from your specific TF. We have a team of four this semester and so they'll be reaching our via email and they'll also be providing you with feedback, both numeric and qualitative on the project, and the aim is to get that feedback back to you at least a day or two before the next project's deadline so that you can absorb that feedback and make any last minute changes to your current code for project one based on any repeated mistakes or decisions you might have made that maybe weren't optimal. So feel free to reach out to me and also to your specific TF via email anytime if you have questions.

All right, so without further ado we move on to things more client side. So whereas thus far, we've looked at XML, and we've looked at SQL, and we've looked at PHP. All of that stuff thus far has been server side, and so we're at the sort of midpoint of the course now where we transition to things more client side. Where we don't do away with all of our server side capabilities, but we start focusing more on writing code that was going to execute in the confines of the browser. So JavaScript is an interpreted language, just like PHP, and what does that mean, to be interpreted as a programming language? Yeah? >> It's read line by line when it's run as opposed to being compiled when it's translated into [inaudible]. >> Exactly. So there's no step as there is with some languages like C, or C++, or even Java, where you have to compile your code from source code into some other representation, often zeros and ones. With JavaScript, as with PHP, you just write it and then you hit reload on your browser, or you hit run on the server, you do some execution of a command that just starts to interpret that file line by line.

The upside of this is that there is quite simply no compilation process, just a little more interactive in terms of the programming. But the downside is some degree of performance. And so on the PHP side, and we'll talk about this in our scalability lecture, there are ways of mitigating that cost of interpreting the same darn PHP file again and again. You can do what's called cache, the up codes, which is a fancy way of saying that the PHP interpreter can save the results of its first interpretation so that the second, and third, and fourth interpretations are much faster but that's purely server side. On the client side you might have noticed in the popular press that when Google comes out with a new version of Chrome, or Fire — or Mozilla comes out with a new Firefox version, everyone's touting their various JavaScript performance numbers.

And that's in particular because so much more of the web these days is based on client side code and the faster that code executes the better the user experience tends to be. And the more necessary it is to have good JavaScript interpreters, especially in older hardware, or mobile phones, or netbooks where you just have fewer CPU cycles and less RAM to play with. So let's take a look at JavaScript, and you should find overall that syntactically it's pretty similar to what you would find in other interpreted languages, among them PHP, but there's a few features and a few approaches that are generally good to be mindful of out of the gate. So, one, in terms of references, these are some okay references. The first two are more authoritative than W3Schools, but you'll find that these links might be some useful reading. But today what we'll focus on are some of the core features of JavaScript, the syntax, most of which you'll find familiar, and then some concrete examples.

And then what we'll do next Monday is focus in particular on Ajax, asynchronous JavaScript and XML, although it doesn't necessarily involve XML anymore. Rather, Ajax is all about using JavaScript to program more dynamic websites, things like Google Maps, and Facebook, that even once you have landed on their homepage they continue to pull in more and more data, whether it's different parts of a map, whether it's status updates from your friends, and that's all thanks to JavaScript and the ability in JavaScript these days to make additional HTTP requests without reloading the whole page. You can instead make other HTTP requests that get back subsets of content and incorporate that content into an existing webpage. So we'll also see the topic of DOM and tree structures recur today and next week as well. So how do you go about writing JavaScript? So JavaScript is not fundamentally tied to the web or to HTML, but it's generally found inside of webpages, or inside of JS files — .

js files that themselves are included in webpages. Now as an aside they're increasingly common is to actually use JavaScript server side. A very popular framework, if familiar, is node.js, which we'll talk briefly about toward the end of the semester, but this is an alternative type of tool that allows you to actually write JavaScript code on a server, or even on your Mac, or your PC, and run it locally without actually needing a browser. So just realize that unto itself JavaScript is a full-fledged programming language but tends to be used in webpages and it tends to be used in between script tags that look quite simply like that. However, if you actually want to put your JavaScript code, such as the stuff we're going to write, in a separate file you can include it in a separate file by a syntax like this. So the script tag takes an optional source attribute, which references a file, and then also a type attribute which informs the browser what the type of that script is. It's almost always JavaScript, but in theory other languages could be supported.

So without knowing much about JavaScript, without actually even seeing any JavaScript just yet, just instinctively, why might you want to be able to embed JavaScript code between script tags in a webpage versus doing it via an inclusion mechanism like this? When might you go one route versus the other do you think? Yeah? >> [Inaudible] a user to look at my code. >> Okay. [ Inaudible Speaker ] Okay. So we can raise the bar to the user seeing my code by putting it in a secondary file rather than my .html file, or my .php file. The one pushback there is that because JavaScript is an interpreted client side language when used in the web, the reality is we can still see file.js, it's just not going to be in the body of the main webpage. But for instance, if we use Chrome, or IE's, or Firefox's Inspector, it's a little debugging window at the bottom, we can absolutely still see the contents of my JavaScript file.

So it raises the bar a little bit but not fundamentally. Jack? >> It lets you use the same JavaScript commands on all sorts of different pages. >> Good. >> Instead of having to paste and copy and all that stuff. >> Good. So much like CSS it can be used across multiple pages if you factor it out into an individual file and then include that file with a link tag in the head of your webpage. Same deal here, with JavaScript if you want to use the same functions or the same widgets in different webpages, whether it's a .html file, or .php file, or whatever, you can factor it out here, include this tag in every one of those pages so that you can then use that JavaScript functionality anywhere you want. So there's just an efficiency mechanism to it.

One, you don't have to download the same file again and again, indeed you get the benefits of caching in this case, because if the browser realizes, wait a minute, I already downloaded file.js before, even if you're including this tag in 20 different webpages of yours the browser most likely is not going to keep re-downloading file.js. By contrast, if you paste it, a whole bunch of JavaScript code between script tags in your HTML, well then that just has to be downloaded again and again, because when you request a file via HTTP you really get the whole thing typically, you don't get everything but the part of it that you downloaded before. So there's that mechanism. And then there's just the design opportunity whereby you don't have to comingle it with your aesthetic markup, your HTML, you can really relegate your logic to a separate file and it lends itself therefore to better designs, certainly for complex projects. So initially we'll start by just embedding everything in our HTML files because one, we don't care yet about performance because these are going to be tiny little scripts at first.

Two, it's a little bit easier to read pedagogically if it's all in the same file. But as soon as we get to the point of doing more powerful things with JavaScript it'll make sense to adopt this model here. All right, so here is JavaScript in list format. These aren't all the keywords but I tossed these up here in bullet form really just to convey the message that JavaScript is quite similar to most of the languages with which you're probably already familiar. So not just PHP but whatever languages you've used before coming into the class. You have break statements to break out of things like loops, or switches. You have do while, you have for loops, you have while loops and the like. You have the return statement, switches, throw, try, and catch, which relate to exception handling, which we talked briefly about on Monday and also is quite popular in Java.

Var for variable we'll see is a little bit different from PHP where we sometimes want to declare explicitly that something's a variable before using it. And functions are actually going to be quite powerful. One of the most powerful and useful features of JavaScript is that it supports anonymous functions and in turn things called closures which for now [inaudible] are going to let us do some fairly sophisticated things like passing functions around as arguments and not just passing variables around as arguments. So this is quite a fancy feature that's now present in PHP and other languages but it's quite a very common paradigm in JavaScript. All right, so some basic syntax. In JavaScript to declare an array, call it a, you can do syntax as simple as this. Var, which means give me a variable, a, which means here's the name of the variable, equals open square bracket, closed square bracket, semicolon. That's it. So this returns to you an empty array. These arrays in JavaScript are only numerically indexed.

So whereas in PHP we saw associative arrays and you could use an array by way of indexing in with numbers, or you could use actual keywords, like the super globals, get, and post, and session, and cookie are all associative arrays. In JavaScript arrays are by definition numerically indexed. So for now, if I want to add things to an array in JavaScript I can do this in a few different ways, I can do a [0] gets equal 'foo'. Totally arbitrary example but it just shows the syntax. If your array is called a, and you want to get at location zero, even if nothing is there yet, you just go ahead and say gets equals bar — or 'foo' in this case, or number, whatever value you want to store there. Same deal for bracket one and bracket two. The one thing to realize here is that they array is actually going to grow automatically for you. You don't have to resize it as you might in a language like C.

You don't have to know in advance how big this array is. You can just keep pushing additional items onto it by knowing at what index you want to place foo, or bar, or baz. So you can actually be a little more deliberate and you can actually call functions. So JavaScript arrays are actually objects, even though you're initializing it with this somewhat shorthand notation of open bracket, closed bracket, they are themselves full-fledged objects in an object-oriented programming sense, and therefore they have methods associated with them. And one of the methods associated with a JavaScript array is a push method, which pretty much does what it says, a.push means call the push method inside of the array, called a, and give it an argument in this case of foo. And that has the exact same effect of just adding foo in this case to the first location in the array.

Why? Well the array is initialized to nothing so pushing an element onto it means you now have a bracket zero location. Call it again and again, you have bracket one and bracket two. So push appends an element to an array, it does not prepend the element. As an aside you may also see syntax whereby you can declare an array with var a equals new Array. So A-R-R-A-Y, capital A, is actually the name of a class, though JavaScript doesn't technically have classes, it's the name of a JavaScript object that exists but the common notation, daresay preferred notation, is just to use the simpler square brackets. But realize you might see that in various tutorials. So good, we have arrays, which means we have the ability to store lists of things and arrays are going to be super useful once we get to Ajax and more generally the retrieval of more data from a server because if you can imagine how something like Facebook is implemented or Google Maps, you're often not just getting one thing back from the server, you want to get back multiple tiles for your map. You want to get back multiple friend status updates. So you want to get back an array of stuff, so JavaScript is going to lend itself to that and you'll see it's a common syntactic tool.

So let's do an example and just to see how we might use this, a very common thing in a webpage is to have a form, like a login form with a username and a password. And at least me, who is a little uptight, it drives me nuts if upon visiting a page and being prompted to login I have to manually click with my mouse inside of the username field just so I can start typing my username. Right? This is completely unnecessary, but if you think about it any of the forms you've implemented thus far without using JavaScript code do not automatically give focus to any given field. You have a text field, text field, and however many more you have declared in your HTML, but if you want to start typing stuff into those text fields you have to click with your mouse just like we did for the past couple of weeks when we've been doing form examples in order to start typing.

So this is not the best user experience, right? If I visited Facebook.com, I'm not logged in, let me just start typing my username and then my password, don't make me click and then type my username and password. So how can we express that idea programmatically? So this is out of context, just in HTML element, called script, inside of which is some actual JavaScript code. So here we go, if document.forms.login .username.value. So, ridiculously verbose and we'll see before long ways to clean this up and just make it a lot more straightforward, but what is this referring to? So in the context of a webpage you have a special, super global, really just global variable called document. And this variable is of type object which means it has stuff inside of it, properties inside of it and possibly methods associated with it.

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So document actually refers to essentially the DOM that we've been talking about. We talked about DOM mostly in the context of XML, but quick refresher, what was a DOM? What is document object model all about? DOM, yeah? >> It's like the structure of the document. >> Okay, it refers to the structure of the document. It was a tree structure, recall. In fact we drew a tree in picture form on a slide that represented a snippet of XML. So DOM is a way of modeling hierarchical data, for instance XML, or HTML, in a tree structure and the roots of that DOM is generally called the document node. It was capital D the last time we talked about it, here it's lower case d, but it's the same thing. So when you're writing JavaScript code inside of a webpage, inside of a whole bunch of HTML, you can access that tree structure programmatically by saying document. And document dot means go inside of this object and access some property inside of it. So for those unfamiliar with object oriented programming, though most of you would be from prior experience, realize that object oriented programming is about having data structures in memory, generally called objects, and objects can have both data and methods associated with them, our properties and methods associated with them.

So, in the case thus far with document it turns out there is a property called forms. Forms happens to be a collection of all of the forms in that document. And there might be zero forms, there might be 10 forms. It totally depends on how many form tags are in that document. So document.forms.login specifically means get me the form whose name is login among all of the forms in the current page, whether there's one or more. Document.forms.login.username, take a guess here, what is username referring to in the context of this story thus far? Axle? >> Well, [inaudible] field of the ID username inside of that form. >> Good, so in this case it's referring to the input called username, happens to be name as opposed to ID in this case. Username field in the form whose name is login in the collection of forms inside of the DOM. So there's a hierarchy to these dots, just like in Xpath and in the simple XML API there was this notion of stepping where the arrow notation that said go deeper, go deeper, go deeper.

Same idea here. Document is sort of the highest level object you have access to, document.forms dives in a little deeper, document.forms.login a little deeper, and .username even deeper. Now, username really refers to the node in the tree that's of type input, so think about how you would make a login form. We've got at least two input HTML elements, username and a password one. So, username is now referring to a rectangle in DOM — a rectangle in the sense that's how we drew it on the screen the other day. So dot value is actually referring to its string value, whatever text is actually in that text field. So let's see this in the context of some actual HTML. Let me go over to the appliance and let me open up a file called form1.html and let me skip the JavaScript code for a moment and focus only on this. So I propose this as one of the simplest possible login screens we could have, and I have a few things going on here. One, I have a form. Two, I have an element called email.

Next, I have another input called password1, and then third I have another field called password2. So you can imagine this being in the context of a registration form. So not necessarily login but registration where you want to ask the user for the same password twice, that'll lend itself to some validity checking in just a bit. And then lastly I have a submit button. So that's it. Well, what if now I want to start getting programmatic access to these various fields, let's do this. Let's open up instead version two of this, form2, and let me propose the following approach. Down here now let's do a registration form. So I've given this form an ID, registration, and I have given it similarly an email field, a password field, another password field, and then a submit button.

But what I want to do this time is when the user tries to submit this form I want to use JavaScript to actually check the values that they've typed in and yell at the user, reject their input if the passwords for instance don't match, or if they haven't filled out this form properly. So we could do this in PHP, think back a few lectures now where we had those various login examples, and if the user did not give me a username or a password we would reject their input and we would check for that server side by checking the value of dollar sign, underscore, post. So we can already check the validity of a user submission server side, so what's the point of even doing form-checking, or validity-checking client side with JavaScript today? >> We can check without even having to reload the page or deal with anything [inaudible] can immediately be done [inaudible]. >> Exactly.

So, with JavaScript now, with client side code we can check whether the form has been filled out correctly immediately client side without even talking to the server and to be more clear, like what is really the value of doing that? Yeah? >> Well, [inaudible] efficiency, you're not sending any, in this case, useless data to the server for validation and then receiving it again. >> Exactly. Really it boils down to performance. With JavaScript on really any computer, any phone these days, it can execute so quickly that you pretty much get immediate feedback for the user, which means they see whatever problems they've created right away. Moreover you're just not adding extra load to the server for really no good reason so you can reduce load on the server which can potentially help with scalability if it has to do less per a given unit of time. So a couple of advantages here. So let's take a look at this form in the browser before teasing apart how we can impose this kind of error checking. So here's the original form, very simple, very ugly, but it has my email field, password film, password again field, and then I also decided I'm going to throw in a terms and conditions checkbox, because this really lends itself now to did the user do everything we wanted them to do, a very common certainly on webpages today.

But notice if I don't do anything here except type in malan, but that's not even a valid email address, a password, I'm going to type in whatever, and then this time it's definitely not going to match because it's only one character, and I'm not going to check this. When I submit version one of this form, which recall had no JavaScript, just the HTML, here's all that happens on the server. I have submitted to a file on the server called what apparently? Yeah? >> Process.php. >> Process.php, which clearly does not much at all. All it does is apparently call the print r function in PHP, the recursive print function, just because I needed a quick and dirty tool to see what was actually sent to the server. In this case that's what I just typed in on my keyboard. Notice that the checkbox value was not even sent because it wasn't checked, but the takeaway here is that I could check on the server whether or not the user cooperated.

Did they give me a syntactically valid email address? Did they give me two passwords that match? Did they check the box? All of those things I can check server side, quite like I did a few lectures ago with checking if John Harvard gave me the right username and password. So all of that you can sort of do already in PHP. But let's do it better or at least more rapidly in JavaScript. So now, let me go back here to version two of form and let me not cooperate this time, too. Let me type in a whole bunch of nonsense and not check the box and click submit and this time what I want to see is an error message. So this is sort of like a poor man's approach to error-checking where I'm just using the JavaScript alert function. Those of you who've been using the web for some time will remember these used to be the scourge of the internet whereby you would visit some website that had spam or wanted you to click links and whatnot, and you would just get peppered with all of these popups, so not the best approach yet, but very easy way of signaling to the user that an error happens.

So we can improve upon this eventually. What's the error? You must provide the same twice. So that came from me it turns out. So let's take a look at the JavaScript code that underlies this particular error-checking. Let me go ahead and open this up. Let me use a different color background here. We're going to go ahead and open up form2.html. And I'll zoom in here. So here again is the html, here's the form in question. Let me scroll over to the right deliberately. What is new or different about this version of the form than last time? Yeah, Axle? >> You intercept the submit and run the function validate on submit, which means that the submission with PHP is not really done until that function is [inaudible]. >> Exactly. So in this case I have on submit, which is a new HTML attribute, and it is attached if we scroll back to the left to the form element, and in this case I've said form, then ID, and then method, and all that other stuff, on submit equals "something.

" What is the value of this on submit attribute? Well it's meant to be actual programing code. Either the name of the function that I want to call, or a string of JavaScript that I want to execute and return some value. So I've literally said return validate open print close print semi-colon, and we've not written much JavaScript yet, but validate is apparently the name of a function, and open print close print means it takes no argument, semicolon means that's the end of the line, though it's not strictly necessary, and return means do something with this value. And indeed, the way the onsubmit handler is defined, if I were to read the documentation, is that if the code between quotes returns true submit the form to the server as usual. If the code between quotes returns false don't submit the form at all. So here seems to be our mechanism that Jack proposed, being more efficient. We can yell at the user right away and give feedback by returning false and maybe triggering an error message but if everything looks good we just return true and the user doesn't even know we've validated their submission.

So it's a nice transparent way of doing something and then maybe intercepting the form submission. So where is this validate function defined? It doesn't come with JavaScript, I had to write it myself, so let's scroll up to the top here. So here's my script tag, and notice below the script tag I have my validate function, and here is how you define a function in JavaScript. Inside that script tag I have function validate open print close print which does what you would expect. In fact the syntax is pretty much identical to PHP; the only difference is involving how you would declare arguments or these default arguments and the like. So function validate means here comes a function called validate. All right, so now I'm using my verbose syntax for now, if document.forms .registration.email .value equals equals "" then I'm going to yell at the user with an alert function which comes with JavaScript, I did not write that one. What it does is it triggers that popup with the message inside of it, and its argument is, "You must provide an email address." And then I return false.

So we didn't see this one but under what circumstances would I have been yelled at with, "You must provide an email address?" Yeah? >> If you didn't enter anything at all in the email field. >> Exactly. If I had not typed in Malan, but I just left that field blank at the very top of the form, I would have been yelled at with this message. And just to be clear, why do I not only call alert, I also return false? Isaac? >> Well, it has to be false or true on submit. >> Okay. >> So it has to return true or false for the sake of the onsubmit handler, but why false, just to be clear? >> So that it doesn't [inaudible]. >> Good. So that the form rather doesn't submit fully.

So it does not initiate an HTTP get or post, sending the form's data to the server. We want to stop that because we're not ready to do it. So we don't want to trouble the server just yet. Okay. So now we have some branching code here. So turns out that if conditions else if [inaudible] in JavaScript pretty much identical in JavaScript to PHP, you need the curly braces if it's more than one line, and in general I would encourage always using the curly braces because of the process called minification. Which we'll talk about later, but that has to do with eliminating unnecessary white space and even shrinking variable names in JavaScript. So generally braces are a helpful thing. So, else if document.forms.registration .password1.value equals equals something, let me scroll over a little further, this is a mouthful, but in English what is this else if doing for me? Axle? >> If the first password field is empty as well it's going to do everything inside the else if.

It's going to alert [inaudible] and then return false to the submit. >> Exactly. So just like before if the user has this field blank, in this case the field is password1, and its value is quote unquote, that is they typed nothing there, then I'm going to yell at them, "You must provide a password," and also return false. Now realize if the user had not left it completely blank but they typed in a space bar character, would this return false at this point? Yes or no? Jack? >> No because it's not [inaudible]. >> Exactly. Because obviously a space is not equal to quote unquote with nothing between the quotes. Its quote space unquote.. This if condition would not catch that and so we would not declare the user's input as invalid yet. So already hopefully you should be thinking, well there's ways of making this better. Right? If the user hits space or space, space, space, or space, space, space just to circumvent my form validation I should certainly be able to detect that too and it turns out there's functions in JavaScript or I could write a function in JavaScript that lets me trim the user's input.

I could trip leading white space, I could trim trailing white space, and that just means throw it away so that even if the user's trying to be difficult or annoying by just hitting the spacebar to get through my stupid prompts I can still catch at least those scenarios. To be clear though, I'm not doing anything like checking the length of the password, I'm not checking whether it has enough alphabetical characters, numeric characters, punctuation characters. There's no like security checking here of the quality of the password. We're just seeing if the user gave us anything whatsoever. What about the third branch here. So else if document.forms.registration .password1.value ! equals ""document.forms .registration.password2 .value then yell at the user as well. And before I spoil the answer, in English what should the error message probably be? What have they failed to do properly here? Yeah? >> You need to enter the same password twice.

>> Exactly. We need to yell something like, "You need to enter the same password twice." Because the logic I just checked was did password one's value equal password two's value? If not, they screwed up again and they typed the wrong thing twice. So, if you've ever been on a website that does exactly that kind of check this is similar to the kind of code that they're writing. Theirs is probably a little more compact, we're doing it the very pedantic way just for now, but the functionality is the same as you would find on a typical website. What's this last block doing? "You must agree to our terms and conditions." Why does the code in that — how does the code in that branch work apparently? Yeah? >> Well it looks for a dot checked on agreement, which is probably the checked box. >> Good. >> And the exclamation mark in the beginning of it says if it's not checked, so then you run the alert. >> Exactly.

It turns out, and this is where DOM again is useful, there's different types of nodes, different types of objects when you parse a big HTML file and an input type whose — an input tag whose type is text has things like a dot value property. By contrast, something like a checkbox is generally useful because it has a dot checked property that is true if it's checked, that's false if it's not. So by saying if !document.forms .registration.agreement .checked, by saying that syntax I'm saying if it is not checked, by inverting the Boolean value then yell at the user accordingly as well. But if they pass the first, and the second, and the third, and the fourth if condition successfully and we have not yelled nor returned false yet, what do I do by default? I return true at the very bottom of the code because they didn't screw up, they typed in everything I wanted them to. Now they didn't necessarily give me a valid email address, they didn't necessarily give me a password that's anything more than a spacebar character, but we've at least raised the bar to just the user rapidly clicking submit and wasting the server's time. We could start to make these more sophisticated and we will but for now that is one of the easiest mechanisms whereby we can introduce some JavaScript code to the site and do something useful for the user's sake.

All right, any questions, syntactically or otherwise? No? All right, well let's take a quick look at form3.html. Actually, let's skip form3 that one's kind of boring. No, we can show it, just not that interesting. So in form3 it turns out there's this key [inaudible] in JavaScript which actually isn't used all that often because there's cleaner ways still to do this. But realize the [inaudible] keyword simply says assume the following prefix for any variables in the block of code that's to follow so that I can just say something like email.value, password1.value, and so forth. But again this is not an oft used feature but it's one step toward making what are right now fairly ridiculously long expressions a little more manageable. So just FYI there. All right, but let's look at form4, which looks a little something like this.

I seem to have a fundamentally different approach here, and before we dive in what is different about this function, this version of validate? Yeah? >> It takes the argument that you call f, which is probably going to be everything before like .command, .form, and everything. >> Okay, excellent. So it looks like this version of validate takes an argument first of all, and that argument's apparently called f, because it's in parentheses there. And I seem to be using f subsequently further to simplify my syntax and more generally because what I'm going to do in this version as we'll see after a short break is that I can pass around even forms themselves as arguments because a form in a webpage, once it's read in by the browser, is represented as a node in a tree, as one of those rectangles that we might have drawn a couple of lectures ago. And so I can pass around a reference to that DOM node, that form node, and indeed this case we'll soon see that the variable f, the local variable f is going to be of type node. It's going to be a form element that was passed programmatically. Why don't we go ahead here and take our five minute break and when we return we'll see exactly how we did this and what more we can do with objects from DOM's.

All right, so the most recent cliffhanger was how does this version four actually work. We're passing this thing called f around, so let's first look at how this function is being called. And notice at the bottom of the form that our html is almost the same as last time except one attribute's value has changed, which is obviously what? Yeah, Axle? >> Well, the [inaudible] function is which probably refers back to that particular node is the DOM. >> Yeah, exactly. So in a lot of programming languages, for instance PHP, and Java, and also JavaScript, there is this special variable called this, which is rather self-referential, whereby if you are in the context of an object or in this case in the context of an HTML element you can specify this and that refers to that context. So the keyword this, in this case, is referring to the element in which we've mentioned this, and if I scroll back to the left what is the tag name in question here? This is the form tag, open bracket, form, action, method, and name, and onsubmit, and there's the beginning of this.

So we have this way now in code to refer to the HTML element that we know eventually will be accessible via that DOM object, that document object. All right, so return validate this. Now let's look at what the implication is of this calling convention. So if I scroll back up to the top where my validate function is defined, recall this is where we left off a moment ago, and the validate function takes an argument f, arbitrarily called f, I could have called it foo, bar, or baz. But thereafter I can specify f.email.value, f.password1.value, and so forth. So one, my syntax has gotten a little simpler, which is nice. Two, I now have the beginnings of more reusable code. I now have a validate function that in theory could validate any form object that has these particular fields of the email, and password1, and password2, so I've begun to at least start factoring out what could be some common functionality. And three, it really demonstrates, hopefully, that via the this keyword we now can bridge the gap between HTML, which is really just markup, and programming code.

And the middleman between those two worlds is this DOM, because the DOM is the result of having parsed an HTML file and built up in memory, in RAM, this tree structure, which JavaScript can then traverse. All right, so the rest of this code is identical, but any questions on this or the passing thereof? All right, so let's take a look at version five here, which is now starting to give us some more interesting UI. Let me zoom in on the left portion of this form, and does anything look new here? Isaac? >> Well, there's disabled. >> Yeah, so disabled is something that's a little new here and there's a couple of ways of writing this. The way I've written it looks a little silly these days but was in vogue when XHTML was being popularized. Could just say disabled with HTML five, but disabled equals disabled [inaudible] that this element, as you might have guessed, should be disabled. [ Inaudible Speaker ] So what's the implication of disabling something like a button? What is this actually going to do? Yeah, Axle? [ Inaudible Speaker ] Okay, good.

So, when you check this, let me go up to this in a browser now. So this is now form5, notice that it's a little subtle here, but notice that now that submit button is a little grayer than usual. All right, well maybe that's just a CSS thing, but let's try to click it and be a difficult user and not fill out anything. I'm clicking, nothing's actually happening because that is in fact disabled. So why is it disabled? Well, this was just a design decision I made in this version whereby I only want the user to be able to submit this form if they have minimally given me what I wanted or at least minimally checked that box. So let me go over to that checkbox and see what happens. Checking now, and notice the difference at bottom left. Every time I check or uncheck that box the submit button changes state and once it is black like this it seems to let me click it again, I'm not actually letting the form be submitted but if I clicked it it would go through.

Thankfully I now have some error checking still in place but more on that in just a moment. So how are we actually doing this? Well, if you were writing the code yourself and you want to tie that button somehow to the checkbox, what was the property on the checkbox element that we want to be checking in order to decide whether or not the submit button should be enabled or disabled? Yeah? >> Dot checked. >> Dot checked. So it turns out in addition to dot checked some elements can have a dot disabled property which literally says should the browser let the user click it or not, is it disabled or not, true or false? So if we go back to version five of this code, let's scroll up here to the very top, or rather, let's first look at this. Notice that on the right hand side here, I agree to these terms and conditions, then I have input name equals "agreement" as before, and now I have another event handler.

So I've done something a little different here. I'm not using onsubmit because I don't want this form to change states when I submit the form because that's too late; I want it to change state when I click on this checkbox. So in English, what does onclick equals "toggle ; mean, or do, do you think? Yeah, Ben? [ Inaudible Speaker ] Exactly, simple as that. Anytime you click this checkbox call the toggle function. There's no arguments passed in, but as the name of the function suggests there should probably toggle the state of what? The checkbox or of the submit button? The submit button hopefully, right? The checkbox you get for free, you just have to check it, the browser will do that for you. So let's scroll up because it sounds like either there's a built-in toggle function or it's something I myself wrote and indeed it's the latter in this case. Here's my toggle function. Pretty straightforward, albeit verbose, if document.

forms .registration.button .disabled then set that same property equal to false, else if document.forms .registration.button.disabled — rather, sorry, else set it equal to true. That was not a condition. So in other words if it is disabled, make it not disabled, else make it disabled. So really I'm just toggling the state of that. I could actually do this a little more elegantly if you prefer. I could just do something like this. I could say document.forms .registration.button .disabled equals that. That's a slightly more elegant way of doing it. Right? I don't really need the if condition, but it's also still a little verbose. But again, we'll clean that up next time. All right, so that's the toggle function, simply toggles the state. What is the validate function doing this time? Well, it's pretty much identical to the very first version. There's no fundamental relationship between validation and the submitting of this form, all I've added this time is this onclick handle — handler and implemented the toggle function myself.

So in terms of user experience now, is this a good enhancement, bad enhancement? The fact that there's this relationship now between the checkbox and the ability to submit the form? Thoughts? Yeah? >> I really don't see anything wrong here but I would alert the user and say you have to click this box [inaudible] nothing's going to happen. >> Okay. Good. So, feels okay in your mind but you should at least be more explicit with the user as to you must click this box in order for anything to happen. And I would agree. Like I think it's okay to sort of really get in the way of the user submitting the form, but frankly I can think of a lot of family friends of mine who would not know what the heck is going on here because it's not obvious that there's this linkage between that checkbox and the submit button. Right? Frankly it might be reasonable instead to just let the button always be enabled but allow the user to click it and then be yelled at, informing them at that point, you must agree to the terms and conditions, especially in terms of accessibility, folks who might have to use screen-readers or the like for reasons of blindness, like disabling things on a webpage that a user, whether blind of not blind, expects to work, is generally not the best practice even though this is not an uncommon paradigm.

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So in short, at least some more explicitness would go a long way. Yeah? >> If you don't even want to enable it can you put some sort of onclick thing so that if it's disabled and someone clicks it [inaudible] check off? >> So absolutely. We could still listen for clicks. So that's a good question. Let's try this because I feel like one or more browsers might actually disable event handlers if it's disabled. But let's see. Let's try this. So, let's go ahead in here and add — I'll zoom in — onclick equals "alert 'testing 1 2 3'; so let's just add this just to see if when it's disabled Jack's idea works. So let me reload the page, and as ever before anytime you make changes to code now that involves JavaScript code you obviously need to reload the page, just like if you make HTML changes you need to see it. And also beware especially when you are including .js files via the script source syntax that we saw earlier.

Be sure also sometimes to hold down the shift key and hit reload or even more dramatically clear your browser's entire cache just because as with CSS, as with other included files, it tends to get cached by the browser and so you might make some change on the server in your JavaScript file, but the browser might not see it because it doesn't realize it's been changed. So, in general a hold-shift reload minimally when reloading files that are included like that. All right, so now, yeah, unfortunately I'm clicking right now on the submit button and it's not even listening. So it seems to be the case here — there we go. So unfortunately Jack's idea does not work, at least in this version of Chrome, because when a form element is disabled via its disabled property or the disabled attribute in HTML, it doesn't even transmit other event handlers like click. >> Now while I don't want to go this deeply into it, could you put it inside a DIV and then have a DIV [inaudible].

>> Yes. Good question. So those of you quite comfortable with CSS, you could for instance put that button in a DIV, or even overlay a DIV [inaudible] absolute positioning or the like, give it a higher Z index and then listen for that. I would agree let's not ever go down that road. That just feels messy. Yeah? >> If you don't want to go down that road, you could also do it all in JavaScript, don't disable it by doing .disabled, do a check in the validate function that checks the state of the thing and then alert the user. >> I agree. Yeah, let's go that road. So instead, leave the button enabled but just add a check to the validate function or some other function that's at least still checking for that error and handling however it wants, but not by just thwarting entire form submission. As an aside, nothing we do today unfortunately is going to make your life easier on the server.

And by that I mean just because we are doing client side validation today with JavaScript and therefore catching errors client side, all of these measures are circumventable by a user or more generally an adversary who's trying to circumvent them. In other words, we absolutely still have to do server side form validation, we cannot trust that the data that's coming from a client just because it had client side validation in JavaScript is actually going to be legitimate. Why is that? How could I circumvent these kinds of form protections? Yeah? >> If you bring up the inspect element thing, you get the developer [inaudible], you could go on the console and then you could inject JavaScript, and then you could essentially just manually submit whatever you want to the PHP. >> Exactly. Anyone who knows how to access more sophisticated tools like the console here could actually execute JavaScript manually or I can even do something like this. If I go into the HTML of this page, let me zoom in and expand the form.

You know what I can do? Why don't I just go in here and just disable — delete this, enter, zoom out, well there you have it. Now my form button is re-enabled. Right? So you don't even have to be a good hacker to circumvent protections like a disabled property, you can just change it right there in that fashion. Or frankly, we can be really sophisticated here and if you really want to be a geek we could do something like, well, if I want to telnet to the server I can telnet — rather, if I want to fake a request, recall we can go back to lecture zero in our HTP request and mimic them ourselves. So telnet is a program that allows you to collect from a client to a server on any TCP port in this case. I'm going to connect to port 80 of the appliance. Now notice I've connected to, and indeed that's the loopback address.

However I could do this to Google, but rather I'm going to do this. So I'm going to say, rather, POST to — oh no, we're using get today. So we're going to use GET /process.php HTTP/1.1. I'm going to say my host is the appliance. And then — woops, notice I already got back my results. Oh, [inaudible], yep. So below that, I won't do the whole example here, what I could continue doing here is the equivalent of passing in — well let's do it. GET /process.php? email-malan&password1 equals asasas&password2 equals z HTTP/1.1, so this is definitely the geekier way of circumventing client side protections. I goofed somehow. Client site php, bad request, ampersand. What did I do wrong? Password. GET /process.php?email equals malan HTTP/1.1. I'm doing something wrong. Okay, anyhow, someone smarter than me could actually just mimic the HTP request to send them from a command line client or from a browser client and actually trick — oh, that's what it was. Okay.

Wait, I can recover from this. Okay, telnet, GET /process.php?email equals malan&password1 equals asasas&password2 equals z HTTP/1.1, host: appliance, I forgot that header, enter, enter. Okay, now I got back a 200 and now look at what I sent to the server. So in other words, if your client doesn't even support JavaScript as my black and white window does not, well then you can completely circumvent those protections as well anyway. All right, so in short, JavaScript good, useful, but it is not a security mechanism. It is really about user experience and decreasing server load at best. All right, so what other kinds of functionality and event handlers do we have access to here? Well, so recall that we had — whoops, recall that we started here. It turns out that we can do a little better in terms of functionality.

So it turns out that like PHP, JavaScript supports a feature called regular expressions via an object called RegEx. So there's some more documentation here, but this is representative of a type of object that comes built into JavaScript. We talked briefly about one already, arrays, and we'll see others, strings, we've used thus far, but regular expression as well is a class in JavaScript or rather an object in JavaScript that allows us to pattern match on various strings. So for instance, thus far we've just been assuming that if you give me anything for an email address it's an email address, but case in point, malan is not an email address, it's missing the @ sign, it's missing a domain name, it's missing a .tld, top level domain. It's just syntactically not an email address. But with a regular expression we could check for that sort of thing. And so we'll do that in just a bit. We'll come back and actually say, you have to give me not just something for an email, you have to give me something that looks like username@domainname.something.

So a regular expression again involves pattern. So we'll see that in just a moment. Strings too have actual properties and methods associated with them and we'll see that functionality before long. And just to give you a sense of the global objects that exist in JavaScript, we have again, arrays, but there's also dates, functions that we've started to use. There's actually a math object that has things like rounding and ceiling and floor and related functionality that's just generally useful to have. Numbers and strings themselves are objects. So in short JavaScript is very much about this paradigm of objects having properties and methods as we've begun to see. In terms of objects, what is an object? An object is just a container for key value pairs. It really boils down to something as simple as that. You can think of it as a hash table. You can think of it as a hash map. You can think of it as an associate of array.

These all have slight differences in terms of implementation but the end result is that an object in JavaScript is a collection of key value pairs; they're generally separated by colons as we'll see in just a moment, but that's just a syntactic detail. So how do you create an object in JavaScript, it's as easy as creating an array in JavaScript. In this case I have a variable called obj, to connote object, and equals, open curly brace, closed curly brace, semicolon. That gives me an empty collection of key value pairs. So there's no keys, there's no values yet. It's like an empty super global; it's like an empty associative array in PHP. How do you assign things to an object? You have two syntaxes available to you. You can say object.key – value and that will create a key called key, and a value of value. Or you can PHP style notation using square brackets and quotes whereby you index into the object at that key, K-E-Y in this arbitrary case, and then assign it a value. So the upside of the first approach, object.

key, is simplicity. It's just very readable and we've been using it all day long so far, document.forms .registration.email1.value. Been doing that the whole time. However, we could've used the square bracket notation it just wouldn't have read as cleanly, and moreover if you ever have keys that have funky characters in them, like spaces or the like, using the quoted notation with square brackets allows you to access those keys. You can't use the dot notation for more complex looking keys. In general though it's probably best to avoid anything complex in terms of a key. How can you create an object and assign key value pairs to it all at once? The very last line here is representative. So if you know in advance that you want to create an object and you know in advance what your keys and your values are, you can simply say, var obj equals brackets and then brackets key: value bracket; and if you want multiple key value pairs just put a comma after the value and do another key: value, key: value, so it's a way of creating an object on the fly with multiple properties.

So we'll see why this becomes so useful. How about event handlers? We've seen a couple, onclick. We saw another. What was the other one? >> Onsubmit. >> Onsubmit we've used a whole bunch of times. It turns out there's a whole bunch of others. Onchange, onfocus, onkeydown, onkeyup, onload, onmousedown, onmouseup, onmouseout, onmouseover, and so forth. Just to infer based on past experiences, when or why is something like onmouseover generally used? Even if you've never heard the phrase until today, when might that be used to detect when the mouse is over something? >> When you hover with the mouse over a [inaudible] field. >> Okay, so when you hover over a mouse with an input field what do you want to happen? >> Well, I really wouldn't want anything to happen, but if you [inaudible] or something you could — >> So you want to hover over a form field? Be more concrete here. We can recover from this.

>> You can — maybe you want to display some kind of info on something you have on your site, like an image, or a file thing. So if you have say a website that lets people download files and if they hover over the icon for the file [inaudible] like the file format and the size and [inaudible]. >> Perfect. Okay. So in contexts where you want to enable the user to hover over something like an image or maybe even a text field, and then you want to provide them with additional information, or additional functionality, then you can use the onmouseover even handler to reveal additional buttons, or to give them the equivalent of a tool tip, a little DIV or something that suddenly appears like a cartoon bubble that explains what it is they're hovering over. And these are omnipresent on the web. If you go to YouTube I think you can hover over most of the HTML5 controls on a player and see what those symbols represent. If you go to Netflix you can hover over a movie posters and then a little window opens and you can see more details about that movie.

Facebook does this a lot, if you hover over a friend's name a little popup opens up where you see some photos of them or their related friends and whatnot. So, being able to detect when the mouse is hovering over something, hugely useful, at least in terms of user experience these days. So how do you use that? Very similar, if you have an element, whether it's an image or a DIV, or whatnot, that you want to listen for the user's hovering over it's just onmouseover equals "something". And we'll eventually see too, there are ways of registering event handlers. That is, telling the browser to call this function when the user does this, using pure JavaScript syntax. You don't have to put any JavaScript code in your HTML in order to achieve this functionality. For now though, we're just using the HTML attributes to keep things simple and to make clear the linkage between HTML and the JavaScript code.

How about onkeydown, or keyup, or onkeypress, which is yet another? When might that be useful? Someone else? Why might it be useful to listen for key presses? Whether the key is going down or going up? Scott? >> [Inaudible] how many times [inaudible]. >> Okay, good. So recording how many times the user types something. Maybe you could use that metric for forms that have only 100 characters or 140, like a tweet or something like that. It might be useful to count how many times the user has typed. Alternatively you could check string length in the value of a field, but counting might be reasonable. What else? >> I've only ever used it once, but when I used it I had like input field and when users typed anything in input field it updated [inaudible]. >> Okay.

>> On the page — well, yeah, on the [inaudible]. >> Okay, good. So if you have a form field where you want to give the user like some kind of automatic preview of it, so they're typing over here but you want them to see what they're typing over here because they're filling out, I don't know, like an e-card or something like that where you want to see a preview on the fly of what they're typing in, well you can use this. Every time they type a character you grab the value of the text field and then you update some DIV element elsewhere, and we'll see how you can update some other DIV or portion of the page using JavaScript as well, but it's certainly possible because you've probably seen websites that do this. Key presses can also be used, for instance, if you want to prevent the user from typing certain characters.

If you don't want any spaces in a field, if you want to make sure that they have — if you don't want them to type in certain characters at all, or maybe it's a numeric field, you don't want them typing letters for whatever reason, you can use onkeydown because what you get with this particular event, you're not just informed that the event happened, you're informed what key the user has typed. So if you detect, wait a minute, that's the letter A, but this is a numeric field, you're supposed to be telling me your account number, you can just return false effectively and say, ignore that particular A the user typed in. So some websites do this too to prevent you from pasting. For instance, Amtrak.com, absolutely hate their website because my email address is in my Amtrak profile and they have this stupid feature where you have to confirm your email address, once and twice. And I'm okay with the principle of confirming your email address because a lot of people, myself included, make mistakes when typing it, but their damn website prevents you from hitting command or control-V.

Which means you can't paste your email address into the field, even if you are absolutely certain that it is correct from the previous field, and that sort of drives me nuts. Other objections I have to people's websites include — actually there's an unnamed website at Harvard that when you're trying to buy something they want a 33-digit billing code, those of you who work at Harvard might know of these things. It's god-awful; I have never memorized a single 33-digit billing code. I copy and paste it. Their website doesn't let you copy/paste a billing code so you have to manually type it out, which I daresay is far more vulnerable to mistakes. But anyhow, what's the pedagogical value of this rant? This is how you would implement such annoying features as those. There are better of uses of this. Right? If you're a Gmail user you might use some of the keyboard shortcuts, C for compose, A for archive, or the like.

How does Google listen for those keystrokes? Well they're listening for A, they're listening for C, they're listening for other keystrokes as well, and they're not rejecting those keystrokes per se, they're actually doing something useful based on them, and that's all because the web browser world is very much event driven. Stuff happens, you hover, you click, you type, and you can listen for all of these events, and this is how you make all the more dynamic of a website these days. How about something like onfocus? What does focus mean in the context of a webpage? Yeah? >> Well if you have an input field and you click it or you tab to it it's going to be focused and that's when you essentially edit the value of it or type something into it. >> Exactly. So in the context of a webpage only one element ever has a focus at even given time.

When, by focus you can think of it very concretely as you've said, like a text area. When you put your cursor in a text area indeed in some browsers it kind of gets highlighted in blue or at least your cursor starts blinking there. That means that form element has focus. If you hit the tab key on a webpage you'll generally see a little blue or whatnot outline move from element to element in the webpage. That means that element has focus and it's just a visual indicator for accessibility purposes as to what you're actually hovering over, because generally you can hit enter or the spacebar at that point to activate whatever it is you're hovering over, or rather, whatever it is that has focus. Blur is the opposite. So when might you care that a user has blurred some form element? In other words, blurring means it's no longer focused, it doesn't mean it gets blurry or anything like that. Isaac? >> Well you can do form validation after they click another element. >> Yeah. Exactly.

It might be annoying and it might not be necessary to check a form for validity with every keystroke, right? And obviously when I start typing my email address it's not going to be valid the first few characters, right? You got to give me some time. So maybe we could instead wait until that form field has blurred in order to check whether or not it's a valid email address. Or we could wait even longer and wait until onsubmit, but onblur gives me that capability. And others, onresize? Onresize is actually a powerful one. If you're building a website that really has some predetermined size and you want that size to grow when the user changes the webpage around, potentially you need to listen for onresize so that you can do some math on your own webpage and make things wider or taller or shorter.

But generally, you get that functionality for free in a browser, but not — that's not the case with various JavaScript widgets. For instance, if you embed a video player, it's generally going to have a fixed width and height. So if you want it to grow over time, if the user resizes the page, you have to listen for a resize event. And then ask the browser, "What's you new height and width, for instance?" All right, so in short, a lot of power there and really the fun in JavaScript client [inaudible] programming tends to derive from an ability to listen to these things. What kinds of things can you do with JavaScript? Well, you can even make CSS manipulations as well. You don't just have to error check and submit HTTP requests. We can do some more aesthetic things.

So for instance, in CSS you have things like borders and colors and underlining and the like. In JavaScript, you can programmatically change the style of a webpage even after your CSS has been downloaded from the server. So for instance, if you think of — try to think of a website where you filled out a form and not only have you gotten some kind of error message if you filled it out incorrectly, sometimes the website will highlight in red all of the form fields that you got wrong, or green all of the ones that you got right. That's actually using CSS, and what that websites JavaScript code is doing, is if it determines that this form field was inputted incorrectly, it essentially adds a class — a CSS class to that input element that gives it a red border, or it literally updates the style attribute of that input element and says, border colon one pixel red solid, something like that if you're familiar with CSS.

So we have this ability in CSS to do this, the one downside is what's in it — what are some examples of CSS properties that relate to Aesthetics? Went right off of you, yeah? >> Border style. >> Border style — border-style. What else? That's actually a really good one. Yeah? >> Radius [inaudible]. >> Okay, radius for like curves around edges, sure. Yeah? >> Color. >> Color, good. Give me some multiple word ones, like border style. Yeah? >> This doesn't have to do with border, but text decoration. >> Good, text decoration, font family, line height — there's a whole bunch of properties in CSS that have hyphens because they're multiple words. Well, guess what a hyphen represents in a programming language like JavaScript? Yeah? >> It might be the same thing as doing [inaudible]. >> Not quite the same as the dot, it's actually more mundane than that. The — the hyphen represents subtraction, like the minus sing.

Like subtract this variable from this variable, line minus height. So the whole world gets screwed up now that no one really thought this through when they invented CSS. So, the reason for this funky looking URL is that there is a mapping between CSS properties and JavaScript properties. So that, for instance, border — what would you say — border style, border-style in CSS becomes border S style in CSS. So you eliminate the hyphen and you capitalize, you camel case the rest of the words in the string. So this URL has a whole list of those mappings, but again, the heuristic is as simple as that. Remove the hyphen and capitalize each of the subsequent, but not the first words. So what's the implication of this? Well, I argue that some of you are too young to remember, but years ago there was a blink tag in HTML, and the blink tag in HTML did exactly that.

It was one of the most annoying features of like, HTML 1.0 or 2.0, whatever it was and you put a blink tag around a word, and it just does this. Even worse, was the marquee tag, which had text going like this across your webpage like a movie banner, right. The web looked awful in the '90s. And we can bring that functionality back thanks to JavaScript now and piece these things together. So, I argue that this function resurrects the blink tag, which it's actually kind of funny. The various browser manufacturers like Microsoft and Mozilla and Apple and the like, like consciously decided to kill that tag and the marquee tag altogether. Few tags have been killed, but those have. But that's okay, we can override their wishes. So here's a function called blinker that's supposed to go through a webpage and any element that has a tag around any text that has a tag around it called blink will have its CSS manipulated programmatically. So we can kind of tie a lot of ideas together here.

So var blinks equals document.getElementsByID "blink." So there's a few new pieces of syntax and functionality here that are representative of very common paradigms. So var blinks represents a variable called blinks. The right hand side is document, that's our DOM object. And recall that I promised earlier that the objects can have not only properties, but methods, well here's one such method. It turns out that in JavaScript, the document object has a method called get elements by name. What does it do? It does that, it gets elements by name, it returns an array of all of the elements, that is all of the tags in a webpage that match the arguments — the quoted argument. So this will literally return an array of all of the blink elements in the page. So what does that mean? Well again, think back to DOM, when we draw a DOM on the screen, it looks like a tree with like, rectangular nodes, even though that's an arbitrary artist's interpretation. So, the array you get back is an array of those rectangles.

They represent all of the blink elements in the DOM. So at this point in the story, blinks is an array of zero or more of those tags. What comes next? Well, we have a variable. Notice that you can declare variables inside of a for loop, just like you can in some languages. So var — for var i equals zero, i is less than blinks.length i plus plus. Well, we have a new feature here of arrays it seems. Blinks.length obviously represents the length of the array, but what is length really? Well, it's a property. Again, objects have properties and array is an object, so blinks.length is the length property of the blinks array object. All right? So what comes next inside of this loop? Well one, I'm iterating over all of the blink tags in the page, and what do I do? If blinks [i].style.visibility equals equals visible, then change it to hidden. So, this is actually a CSS thing. If unfamiliar, a element on a webpage can be "visible" or it can be hidden and those CSS properties do exactly that.

So by default, all elements are visible. So this is saying, if the ith blink element has a visibility style of visible, what am I obviously doing to it? What's — >> In the if statement? >> In the if statement. >> Checking whether it's visible. >> Okay, so I'm checking if it's visible. And if it is visible, what do I do? >> You — you hit [inaudible] you make not visible. >> I make it not visible. I change that same property to hidden. So, what is this exposing to us? So it turns out in JavaScript, any time you have an object that happens to be an HTML element — and a blink tag is an HTML element, it has a style property. That's one of the things you get for free. So if you're accessing elements in a DOM, like a blink element, that blink element has automatically a dot style property.

That dot style property is essentially equivalent to the style equals "attribute" that we could have put in the HTML. So it's the CSS stylization for that element. We can now say change the visibility attribute to be hidden instead of visible, else, do the opposite, that's all we're saying in the else. So just to be clear here, let me bring up a little scratchpad for a moment. Just to be clear, if I had done this in my style sheet or in my style tag, I might have something like this. Blink — this is CSS now, I might say visibility visible, so that's the default, so it would be kind of silly for me to put this in a style sheet, but that's what I've just done. What the if condition is doing is it's changing this effectively to this — on off, on off. So what's the aesthetic effect? Well, let's take a look.

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Let's go back to the browser and let's pull up this one, which is in a form example, this one here is the blink.HTML and you can see what it's doing. Right. Hideous tag, you know, it's not all that annoying here, but if you see — actually you can still see remnants of this on the web, like really crazy looking websites that are trying to use the blink tag, but it just didn't work. So let me scroll up here because this is actually kind of interesting. Let me zoom in over here and you can see thanks to Chrome exactly what I'm doing. >> I have a question. >> Uh-huh. >> Is — are — on this page, are you using the same script you had in the PDF? >> Same — yes. >> Because in the PDF, it iterated over the number of blink elements, which in this case is just one. So wouldn't that be one — one iteration, just [inaudible]? >> Ah, correct. So, I'm using the same blinker function, but I'm calling the blinker function a little differently.

So let me open that up. Let me go into blink.HTML, here's my blink function, but there's one additional line of code that's necessary here. What is this very last line of code in my script tag doing apparently. Jack? >> It makes sure that it won't [inaudible] again until, I guess [inaudible]. >> Good, although let's not put it that way because it doesn't really make sure that it won't call it, because it's only going to call it — it's not going to call it at all based on the code I have here. >> It makes it call it once in [inaudible]. >> Good. So, window.setInterval is a method of the window object, which we've not seen before, but it's another one of these special top level objects. Just like you have document, which represents your actual document, window is a different object that represents like a chrome on the screen, it lets you do things like window size redetection and so forth.

So window has nothing to do with the HTML, really has to do with the browder — the browser, the — the rectangle that is your browser window. So window.setInterval does that. It sets an interval, which means call the following function again and again, every some number of milliseconds. So blinker is the name of my function, 500 means 500 milliseconds or as Jack said, half a second. So that line is simply saying, set an interval of 500 milliseconds whereby every time that interval happens, every 500 milliseconds, call the function called blinker. Now notice one key detail. I have not written blinker as a function call, I have written it as a function name. So it let me go here. This is wrong. I've just added to parenthesis, open paren and closed paren, that is wrong, but why? What's the difference? Or what would happen in this case here? Jack? >> It would try to see if blinker — it would actually call blinker right there and see if blinker has a return value that might be [inaudible] what it wants.

>> Exactly. So in this case, just like in all of the examples we've done this far, blinker open paren and closed paren means call blinker, right. There's nothing fancy there. The problem with that is that as [inaudible] that will effectively call blinker once and that's it. But what you're trying to do here is apparently set the interval of 500 milliseconds to call whatever the first argument is. Now does blinker return anything? It actually doesn't return anything at all, which means you have nothing to call. When it returns, it's not returning function, so nothing's actually going to happen on subsequent passes. So this is actually incorrect. By contrast, if I just say blinker without the parenthesis, that means pass in a function pointer, so to speak, pass in a reference to this function because what the window.setInterval timer will do is add the parenthesis effectively itself to call that function every few seconds. So we can do something with this. Here's how you make a really annoying webpage. But just [inaudible] here, let's hammer this home.

So, let me go ahead and do this. Let me delete my blinker function and let me do — create another function, let's call it annoy. And we're going to have the annoy function say alert hi. All right, so if I do this here — 500 milliseconds will drive us nuts, so let's actually to like 2000 milliseconds, so 2000 seconds, let's do annoy — actually let's do it five, otherwise we'll really go nuts. All right. So this is effectively going to do the same thing, but we'll demonstrate an example this way of another feature in JavaScript that I alluded to earlier. So now I'm going to reload this page, the blink's going to go away because we deleted that. And here we have hi, okay good. I'm going to click okay. Hi, okay. I'm going to click okay. All right, we don't have to go through this forever, but this will go on forever, so that's fine.

Actually be careful of doing things like this because if you do it too fast you have to like force quit the whole browser because you can't actually stop. Anyhow, let's go and rewrite this slightly differently. So, this — oh, my God, all right. So here, we can fix this. Let's comment this out, let's go back here, reload, okay. So now we've — now that should stop. All right, so what don't I like about this? Well one, the functionality is kind of annoying, but two, it's kind of stupid that I defined a function only so that I could reference it the next line down. Right. This is kind of a waste of your name space, so to speak. In other words, if you're only going to write a function once, and then call it in one place, it's not really fundamentally necessary to give it a name, right. The only reason it has a name at the moment is because I want to define it up there and then call it down there. But what if I could do both of those at once, especially if this is a throw away function that doesn't need to be called by anyone else or even by me anywhere else? So what if I instead do this? And the syntax is going to look a little weird for a moment.

Let me do this. Instead of defining a function that way, let me get rid of its name, and let me say var annoy equals function semi-colon. So what have I done here fundamentally? So the functionality now is still identical, it's just that now rather than have defined the function called annoy with the previous syntax, this time I've said, give me a variable called annoy coincidentally, and set it equal to a function. So this is where JavaScript starts to get a little powerful and a little different from some languages. You can have what are called anonymous functions whereby you define a function, but you don't give it a name, but you can assign that function to a variable so that you can still call it. So at the end of the day, this is identical to what I just did, but the takeaway now is that JavaScript has a function called function who's return value is, if you will, a pointer to itself, is a reference to itself.

More concretely, it's the address in memory of where that function lives, though that's kind of an overstatement. But in other words, it's some return value that will subsequently — subsequently let you call that function or pass it around to be later called. So in other words, variable is still called annoy, but the type of that variable is not an int, it's not a float, it's not a string, it's a type function. So functions are, as they say, first class objects in JavaScript, which means you can pass them around in memory just like you could [inaudible] typical variable like an int, a char, a float, a string and so forth. So, what's the next step here? Well, that would work, but this is still kind of silly. Now I have a variable called annoy that I'm only using once, feels like I can do better and I can. Let me adopt this same idea and rewrite setInterval as follows. I want to define an interval whereby every some number of seconds, the following function is called.

Done. So this is an incredibly common technique in JavaScript whereby now I have truly defined an anonymous function, but that's fine because I have informed setInterval of the address of that function, all right. Because the function function — sorry that that's a little confusing, but the function function returns effectively the address of the function that was just created. Doesn't have a name, but that doesn't matter because setInterval doesn't care what its name is, it only cares about that reference that can be passed around. Now this is a little cryptic, it would have been nice if the interval — if the number came first frankly, but whatever, someone chose to put the number second. So I still need the comma, but notice how I wrote this all on one line. I don't even have to be that anal, I can instead put this over here. I can indent it. I can then put the curly brace on the previous line, so this is actually a very common way of seeing a function. If I really want, I could put this down here, but it's actually very common in JavaScript to put the curly braces on the same line as the function.

So this actually looks a little cryptic, but it's just the natural progression of the various examples we did leading up to it. So in English, window.setInterval takes two arguments, one is the address of a function. The other is number of milliseconds. Doesn't matter how we pass in that alert function, we can still pass it in — we can pass it in anonymously. So what we've just done is used an anonymous function or something more fancily called a lambda function, which is simply in this case a function that has no name. But it will still work, so let's try this. Let me go back, save this file. Go back to the browser. And actually let's do it, instead of every five seconds, let's do every two. So the story gets told a little more quickly.

Hi, hi, hi, browsers thankfully have gotten smarter, so let's ignore this one. Back in the day, you would've gotten another hi, another hi, another hi, another hi, another hi, at which point you realize you should — really shouldn't have visited that website. Java — Chrome at least, is better about managing duplication, but that is not necessarily a given. All right, so any questions on anonymous functions? No. If you really want your mind to be blown, let me do one other thing. This is really just to show off and to get you thinking about more sophisticated syntax. Let me go ahead and get rid of the interval and just do var f gets function and we'll do the alert thing like this. So this is similar to before. I'm not called f yet, but if I wanted to, I could do this — f open bracket, all right. So let me go ahead here. Go to my appliance, reload — whoops, okay, reload.

Oh, there it is. It happens immediately this time because there's no interval, there it is. Every time I reload, it starts. Hi, but it just happens once. So that's interesting. And if I really want to be fancy here, I can do this, just so you've seen this syntax because again, you'll see it in online tutorials perhaps. If you have a function like this, you can actually wrap it in parenthesis and then put the trailing parenthesis like this. So what is this going to do? And let's change the text just so you believe me that we're actually making some changes here. This is crazy looking syntax and this is not representative of like JavaScript coding. Don't — don't take this too much to heart, but what's this going to do Axle? >> I think it's going to call itself.

>> Good. Yeah, it will call itself in this sense. Not so much recursively, it just means that the thing between the first set of parenthesis from here to here is a function reference. The fact that I'm putting parenthesis at the end means that I'm actually going to call it and indeed, yes, I did just call that function. So what's the takeaway? Again, it's not meant to, [inaudible] sort of overwhelm with the very syntax, but it's just to suggest that functions are really no different than any other types of variable that you can define. You can pass them around. You can invoke them by using the parenthesis and this is going to be incredibly common in various JavaScript libraries that you'll use whereby you'll need to tell a library like, jQuery, which is a popular JavaScript library, what function to call when an event happens, like blinking or rather like blurring or focus or on mouse over, or the like.

And indeed you can create these functions on the fly as we just did. So what other features will we be able to have access to here? So the blink example was just really meant to give us a sense of how we can override CSS and nonetheless, navigate a DOM, but libraries give us a range of much sexier functionality than the blink tag. So here just a few of the most popular JavaScript libraries out there today, jQuery is probably the most JavaScript — the most popular JavaScript library. And frankly, most people these days almost [inaudible] the two. If you are a JavaScript programmer, you minimally know jQuery, and indeed a lot of the syntax we use today is sort of date in that you don't have to type out document.forms.registration. I mean, even I was getting bored saying [inaudible] phrases that were that long, so libraries like jQuery as we'll see will dramatically simplify the type of code that we need to write. Bootstrap is a very popular library that Twitter developed that makes it much easier to do things like icons and tool tips and buttons and menus, and the like.

I'll pull that up in just a moment, Ext JS also similarly has widgets and the like, MooTools and YUI have the same. So what do I mean by this? Well, if we go to bootstrap, so let me search for bootstrap CSS. Go to the first link there from Twitter. It's incredibly common these days to not reinvent the wheel yourself and implement every single little feature of your website. Implementing buttons is not a very interesting problem. It might have been once, but not to do it for every darn project that you write or menus or tooltips, or these things I rattled off earlier. So just to give you a sense of what bootstrap comes with, you get a whole bunch of features. For instance, in bootstrap simply by downloading their JavaScript code, which is open source and available, you can for instance, create a modal. A modal window is a blocking window that prevents the user from doing other stuff while it's open. Let me click demo modal, something like this where something drops down and then grays out the background.

Notice here that I have a window over it, that's a modal. Well, I can open a modal in bootstrap literally by just calling one line of code like this. We'll see more on this kind of syntax before long, but dollar sign tends to represent jQuery and .modal here is a method associated with a div who's ID is myModal, but more on that to come. What else can you do with a library like this? And again, this is representative of all sorts of things. Let's scroll down to the next example. So things like this, little menus that you might want to hover over. This is all very doable using a combination of JavaScript and CSS. In fact, our simple blink example was a step toward using code to manipulate CSS and open things like menus and the like, but all of this can be done with a JavaScript library. So knowing how to call these functions gives you a whole bunch of functionality. Let me scroll down to — oh, let's see. Tabs are very common, so let me zoom in here, implementing something like this is very easy with one of these JavaScript libraries, you simply have to configure it using code like this here in red and black.

Let me scroll down further — tooltips, so I alluded to these earlier, like in Netflix. Which event is this feature apparently using? [ Inaudible Speaker ] Yeah, so this is probably the on mouse over event, right. Because as soon as I move my mouse over what's probably span tag or an anchor tag, something else is popping up on the screen. So that's possible here. This is another similar one where I get an even bigger box. This is even more Netflix style, where it's a whole window of text. In this case, it's a little boring, but it's [inaudible] just black and white, but that's possible. Alerts, we don't have to use the browser based alert, indeed the JavaScript alert function, generally don't use it. One, it looks different on all different browsers and two, it usually says something stupid like, "The webpage at CNN.com says," like it's just not very professional typically using the built in one, so it's fine for class and debugging and diagnostics, but something like this is a little — let's see, is a little more compelling. Popover — oh, not popover, alert — oh, this example is not working. That button should be clicking — oh, maybe it's because I already clicked it, no, let's see.

Okay, not working as I expected there, for just a moment. And then you have these kinds of toggles. So in short, by understanding how you can manipulate things like objects and pass functions around and the like, it gives you access to such a broader range of functionality so that really your project one or project two can be all the more engaging as a result of being able to tie these various widgets together. All right, any questions on objects and libraries and the like. And again, we'll come back to things like that in the next project. All right, so then a word on some other features of JavaScript, especially as you look ahead to the remaining projects. So there's this notion programming of static code analysis, a fancy way of saying, checking your code for correctness for some tactic validity and consistency with certain rules. When it comes time to actually write your own JavaScript code, realize that — and the emphasis to be clear of, project two will be on JavaScript, realize that you can go to a website like this, jslint.

com, paste in your JavaScript code and it will look for common mistakes in your actual JavaScript code, much like the W3C's validator can do the same for HTML. So do take advantage ultimately of something like that. But I've eluded to this concept a couple of times and it's an interesting one when it comes to security. You've probably gone to a website and seen crazy looking JavaScript code that frankly we can't read ourselves tonight, even though you pretty much have seen the basic syntax and fundamentals of JavaScript. Case and point, let's go to Google.com, let me view the page source here and what does this now look like? We pulled this up a few weeks ago now. This is all JavaScript code, right. But frankly, you know, I can't really make heads or tails of most of that code, why? Well, Google has compressed it or minified it as tends to be the buzzword, which means they eliminated white space that doesn't need to be there. They have shortened variable names from something like foo to just f. In other words, they trim function names and a whole bunch of other transformations. Why? Like why has Google chosen to make their code so unreadable? >> Well, the typical user is never going to be looking into finding out why the Google page works like it does.

And on Google's end, this is less that it has to send out to every user that [inaudible] page — >> Good. >> — Google [inaudible] site, they need to [inaudible]. >> Yeah, so a couple of reasons. Like one, like you know, normal humans like us, we don't need to see this, so it doesn't need to be pretty printed or well formatted with good variable names. It needs to be designed well underneath the hood, but aesthetically doesn't matter to us what it — what it looks like. And then performance, as Jack says, if you think about — it's kind of remarkable. If Google gets maybe a billion hits per day on their homepage, imagine if a programmer simply accidentally hit the spacebar at the end of the file leaving a single character of white space, not a big deal, right. We've all done it. We all do it quite often probably. But if a billion people are going to download index.

HTML today and this programmer hit the spacebar, how many more bytes of traffic is that for Google to transfer. Yeah. >> Gigabyte. >> It's a gigabyte, right, like a single space character for someone as big and popular as Google, it means they're going to have to transfer an additional gigabyte of information for no good reason. My God, what if he hit the spacebar twice, that's two gigabytes, 10 times, that's 10 gigabytes. So there's very real world implication, certainly for the Google's and Facebook's of the world. But even for your own website, like why bother wasting bandwidth on things like that when you can compress it once, still keep an original copy for your developer so that humans can actually read it and update it later, but the browser doesn't actually care. So this is a very common technique for that reason. And what's a third potential reason for wanting to make your code look more like this? Jack? >> If you want to kind of deter competitors from trying to find out how and why your code works. >> Good. So if you want to deter competitors from how and why their code works, you can try to make it look a little more like this, very easily.

Now realize that the smartest of people, the smartest of adversaries, the smartest of competitors, they can still figure out what your code is doing. In fact, there exist tools that do kind of the opposite of compressing it. They will make it much more readable. It can't obviously recover variable names. If you have changed the variable from foo to f, these — these decompilers as we might call them, are not going to be able to resurrect information that's been thrown away, but we could absolutely with, you know, a few minutes or in a couple hours time figure out how most of this is actually working. So all you're really doing is raising the bar. But that's probably enough, right. Because if your — you're worrying about your competitor, and your competitor is smart enough to decompile your minified JavaScript code and figure out how your code works, frankly the person is probably smart enough to just go implement it from scratch without wasting time figuring out how yours works. So, all your trying to do is raise the bar to adversarial attacks like this. So to be clear with something like JavaScript, with something like CSS, with something like HTML — you cannot, cannot, cannot, cannot encrypt it or prevent other people from seeing it.

That's just not the way the web works. There exists tools that claim to do this, and there exist tools that do actually do this. These are a few that do indeed minify or compress your code, but they do not fundamentally change anything because even something like the middle one, which effectively compresses your — rather encrypts your code, the thing about encryption is that if you have a secret key with which to encrypt code, what do you need to decrypt it? [ Inaudible Speaker ] Oh, yeah, well maybe not generally the public key, but the private key, but if we were using what's called symmetric — symmetric key cryptography, the same key. So how do some of these products that literally can — or you can find being sold online to encrypt JavaScript. How do they work? Yes, they do encrypt your JavaScript code, but they also include in the encrypted file, guess what? The key, right. So it's idiotic, right.

It's not fooling anyone who is sufficiently advanced, so just realize that the short answer protecting your intellectual property when it comes to JavaScript, HTML, CSS, all you can do is raise the bar to people figuring out what you're doing. But that should be okay in theory, right. If the value of your company or your product is that you have users and you can't just steal those by viewing source, or you have actual databases and data and interesting stuff going on server side, that tends to be where some of the real intellectual property is, more server side than client side. So don't be misled into thinking that these will keep your code safe, all it will do is save you bytes and raise the bar, which those of themselves are certainly still valid endeavors.

So let's actually try one of these. Let me go to this one here. This is just a smart guy who's put this together. So these are actually reputable ones, the ones I put up. If you're trying to be sold any such tool, I wouldn't pay for it. Let me go into one of tonight's files so that we can dig up, let's see — let's go into tonight — whoops, let me pull up, let's see, will that work? All right, let's just try it, we don't need a whole example, we can do it by hand. So let's see — var annoy gets function, let's see if this works — alert hello closed brace semicolon, okay. So here we go. We're on this packer site, I'm going to click pack. Okay, so here's what I've just done to minify my code. You know, it's still quite readable frankly, we could all probably figure out what this piece of code now does.

But what did it obviously remove? [ Inaudible Speaker ] Okay, so white space is all it removed. I think this site has a few options here just to make this a little more interesting. We can shrink variables, so I've just check the box at right that says shrink variables. Let's pack this again. Now that really didn't do anything this time. So let's do base62 encode, what does this do? Okay, so this is what these sites tend to do when they claim their encrypting your code. So this is the same thing. What is that doing? What does that program do? [ Inaudible Speaker ] It prints hello, right. That's all it does. Now, all of this though can be reconstructed and frankly, this just, it's kind of a cute little [inaudible]. So notice this guy has define an anonymous function with the function keyword that takes six arguments, p, a, c, k, e, r just because he's being clever with his name.

What is he passing in? Well, if we actually parse the cell, one of these arguments is apparently this variable expression e equals string, the next is this if condition, one of them is this split here where clearly this is relevant to the original code. Again, all we have done is obfuscated the code, made it look complex, but at the end of the day, a smart adversary could just execute this function and see what the result is and piece it back together. Any questions? Whoops, sorry. Whoops, sorry. Questions? All right, so let's go ahead and wrap there. Myers here for section, which will dive a bit deeper into JavaScript, Monday too, we will introduce the server side of JavaScript, whereby we will actually have the ability to perform and calls, additional htp requests that will allow us to get more data from the server and do all the fancier things while still using the same client side technique to actually update and not just reverse the DOM and the local webpage.

So more on that to come. I'll stick around for one on one questions..