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Learning about Javascript

Tuesday, October 1st, 2013 by Servage

In this section, I’m going to introduce you to JavaScript. Now, it’s possible you’ve just recoiled a little bit, and I understand. We’re into full-blown “programming language” territory now, and that can be a little intimidating. We’ll start by going over what JavaScript is—and what it isn’t—and discuss some of the ways it is used. The majority of the chapter is made up of an introduction to JavaScript syntax—variables, functions, operators, loops, stuff like that. Will you be coding by the end of the chapter? Probably not, but you will have a good head start toward understanding what’s going on in a script when you see one.

What Is JavaScript?

If you’ve made it this far in the book, you no doubt already know that JavaScript is the programming language that adds interactivity and custom behaviours to our sites. It is a client-side scripting language, which means it runs on the user’s machine and not on the server, as other web programming languages such as PHP and Ruby do. That means JavaScript (and the way we use it) is reliant on the browser’s capabilities and settings. It may not even be available at all, either because the user has chosen to turn it off or because the device doesn’t support it, which good developers keep in mind and plan for. JavaScript is also what is known as a dynamic and loosely typed programming language.

What it isn’t

Right off the bat, the name is pretty confusing. Despite its name, JavaScript has nothing to do with Java. It was created by Brendan Eich at Netscape in 1995 and originally named “LiveScript.” But Java was all the rage around that time, so for the sake of marketing, “LiveScript” became “JavaScript.” Or just “JS,” if you want to sound as cool as one possibly can while talking about JavaScript.

JS also has something of a bad reputation. For a while it was synonymous with all sorts of unscrupulous Internet shenanigans—unwanted redirects, obnoxious pop-up windows, and a host of nebulous “security vulnerabilities,” just to name a few. There was a time when JavaScript allowed less reputable developers to do all these things (and worse), but modern browsers have largely caught on to the darker side of JavaScript development and locked it down. We shouldn’t fault JavaScript itself for that era, though.

What it is

Now we know what JavaScript isn’t: it isn’t related to Java, and it isn’t a moustachioed villain lurking within your browser, wringing its hands and waiting to alert you to “hot singles in your area.” Let’s talk more about what JavaScript is.

JavaScript is a lightweight but incredibly powerful scripting language. We most frequently encounter it through our browsers, but JavaScript has snuck into everything from native applications to PDFs to ebooks. Even web servers themselves can be powered by JavaScript.

As a dynamic programming language, JavaScript doesn’t need to be run through any form of compiler that interprets our human-readable code into something the browser can understand. The browser effectively reads the code the same way we do and interprets it on the fly.

JavaScript is also loosely typed. All this means is that we don’t necessarily have to tell JavaScript what a variable is. If we’re setting a variable to a value of 5, we don’t have to programmatically specify that variable as a number.

As you may have noted, 5 is already a number, and JavaScript recognizes it as such. Now, you don’t necessarily need to memorize these terms to get started writing JS, mind you—to be honest, I didn’t. Even now my eyes gloss over a little as I read them. This is just to introduce you to a few of the terms you’ll hear often while you’re learning JavaScript, and they’ll start making more and more sense as you go along. This is also to provide you with conversation material for your next cocktail party! “Oh, me? Well, I’ve been really into loosely typed dynamic scripting languages lately.” People will just nod silently at you, which I think means you’re doing well conversationally. I don’t go to a lot of cocktail parties.

What JavaScript can do

Most commonly we’ll encounter JavaScript as a way to add interactivity to a page. Where the “structural” layer of a page is our markup and the “presentational” layer of a page is made up of CSS, the third “behavioural” layer is made up of our JavaScript. All of the elements, attributes, and text on a web page can be accessed by scripts using the DOM (Document Object Model). Using JavaScript. We can also write scripts that react to user input, altering either the contents of the page, the CSS styles, or the browser’s behaviour on the fly.

You’ve likely seen this in action if you’ve ever attempted to register for a website, entered a username, and immediately received feedback that the username you’ve entered is already taken by someone else. The red border around the text input and the appearance of the “sorry, this username is already in use” message are examples of JavaScript altering the contents of the page, and blocking the form submission is an example of JavaScript altering the browser’s default behaviour.

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