Intro to jQuery

Demo starter files

Javascript, The Language

Where HTML is used for the structure and content of a web page, and CSS is for the layout and appearance, javascript is used for behavior and interactivity.

Despite its name, javascript actually has very little in common with Java, which is an entirely separate language.

Javascript is a scripting language which (among other things) means that it's interpreted and not compiled. Compiled languages must be converted into machine code to run, but interpreted languages are read as normal plain text files - just like HTML and CSS.

Javascript is a client-side language. This means that script files are downloaded along with the HTML, CSS, and other assets of a given web page. The code is interpreted and executed by the browser, so your computer does all the thinking and processing. Server-side languages on the other hand exist– well, you can guess. They're run by the server and are almost always invisible to the client.

Because javascript is client-side, and written in plain text, it's very accessible. Just like HTML and CSS, we can easily take a look at the scripts that are running on a given web site.

Other Uses

The primary function of javascript is interaction on web pages. Because it's the only language available in the browser, it has become widely used, fast, and mature. And since it's familiar to so many developers, javascript is now used in many places well beyond web pages.

Node.js is an implementation of javascript for dynamic server-side applications and build tools. Adobe products use javascript for creating custom actions. There are even frameworks that can convert javascript into native apps for iPhone and Android! Javascript is everywhere.

Syntax

Javascript syntax is a little bit more complex than HTML and CSS, but the basics are very similar.

  • Whitespace is ignored, very much like HTML and CSS
  • Statements in javascript always end with a ; similar to CSS
  • It's also case sensitive, so pay close attention when naming your own variables and functions!

Because whitespace is ignored, programmers are encouraged to space things out as much as needed to make the code readable.

That whitespace does take up a little bit of extra file space, so it's very common for programmers to minify scripts when they're all finished. Minifying means removing all of the whitespace, which reduces the size of the script file, but also makes it virtually impossible for humans to read. We won't worry about stripping the whitespace from any of the scripts in this class.

Most developer tools have a "pretty-print" function, which automatically reformats minified script files back into more human readable spacing.

jQuery

jQuery is a widely used javascript library that simplifies DOM manipulation and javascript syntax. It's fast, lightweight (loads quickly), and extensible, meaning anyone can author plugins that add additional functionality to the library.

Ok, I think I just threw a bunch of potentially new terms at you.

The DOM, or Document Object Model, is basically the same as the structure of the HTML. It's also often referred to as the document tree. If you haven't thought of it this way, the HTML of a web page is often visualized as a tree. The <html> element is the root element, and each additional element is a branch, with more elements branching off as we get further into nested elements.

A library, or framework, is a large collection of functions and definitions that make a certain tasks easier to do in a given programming language. It's basically "boilerplate" code that saves you from reinventing the wheel every time you start a new project.

A plugin is almost the same thing, only smaller and more specific to a certain task.

An analogy might be helpful here too.

Say you're starting a new project - to build a house. That's a lot of work to do completely from scratch! You have to design the floor plan, dig a hole for the foundation, build the foundation brick by brick, and go from there. A library or framework is like a pre-constructed foundation. In fake pseudo-code let's call this "import foundation". With this you don't have to do any digging, and a logical size and footprint are already laid out for you (probably by someone who knows even more about building houses). This lets you skip a lot of the hard planning and base work, and get right to building the actual house, which is much more important to you.

A plugin in this analogy would be like "import kitchen". This is a smaller, single-purpose room that comes with a pre-installed refrigerator, range, dishwasher, etc. You can still customize things like the cabinet materials and appliance colors, but again you save a lot of time by not building them yourself.

The additional benefit of using libraries, frameworks, and plugins is that they're often open source. This means lots of people have used them, and experienced people have likely contributed to making the code fast, compatible, and well documented.

A Little Background

jQuery is a javascript library that simplifies the use of many common tasks in javascript. Tasks that normally take several lines of not-so-friendly "vanilla" javascript can be accomplished with significantly less typing, and better readability. Hence the jQuery slogan - write less, do more. It was first launched in 2006 by John Resig, who's still on the team of jQuery's developers. It's now the most widely used javascript library in the world.

The jQuery web site has really great documentation of everything at your fingertips when using jQuery.

  • Here's the starter tutorial on how jQuery works. We'll do something very similar to this together, using my demo files, to get you up and running.
  • learn.jquery.com has textual explanations with sample code that can walk you through the basics and best practices.
  • Once you're familiar with the basic syntax, you'll probably spend more time referring to the API Documentation. It has definitions of all of the methods, events, etc. available in jQuery, with simple demos showing what they do.

Diving In

Great so how do we get started using it?

  1. Link to the jQuery library in your document <head>
  2. Link to a script file of your own, or embed your script between <script> tags in the HTML. This is the same idea as linking to a stylesheet or your own, or typing CSS between <style> tags.

To start using jQuery, it needs to be loaded in your html, like a style sheet or other external resource. Luckily we don't even need to download jQuery to do this. We can get jQuery from the Google CDN.

There are a few advantages to sourcing jQuery from Google instead of your own server. It's free, it will often load faster than from your own server, and many users will already have it cached in their browser thanks to other sites that use it.

The jQuery core library doesn't include all of the superfluous animation effects you might want to use though. Those are part of jQuery UI, which is an add-on library. jQuery UI includes another .js file, and a style sheet. We will also source these from the Google CDN for this demo.

Once you've linked jQuery, which sets everything up, you can also get started typing your own script file(s).

Now may be a good time to switch over and do the demo!

Here are some helpful terms to know when getting started, and reading the API Documentation:

  • jQuery Object - the DOM element(s) you're targeting inside $().
  • Method - a method is anything you can do with an object. jQuery has tons of methods for manipulating objects.
  • Function - a chunk of code to be executed. Functions can be called right away, or defined with a name and reused later.
  • Event - something that happens on the page. Mouse clicks and hovers are events.
  • Event Listener - a function that is triggered when an event happens.
  • Callback function - a function that executes when another function finishes.
  • Chaining - One of the most powerful aspects of jQuery. Methods can be chained together to happen in sequence, similar to a callback function.

About Animations

jQuery is great for very simple animations, but it has trouble with more advanced animations, and with animating lots of things at once.

CSS transitions and animations are better optimized to use the computer's GPU, and spawn separate threads for certain calculations, making them far more efficient. Don't be afraid to use jQuery to animate things! But it's a good idea to use CSS transitions if you have the option.

There are other javascript libraries aimed specifically at animation that are more efficient like CSS. GSAP, velocity.js, and d3.js are some of the most popular. Velocity is even a drop-in replacement for jQuery's .animate() function.

Very advanced animation and visualization is all done in javascript using libraries like these.

Plugins

Plugins for jQuery (and others that don't require jQuery at all) can be found on the Resources page.