Tuesday, July 13, 2010

Things your parents never told you about JavaScript - Apply invocation pattern

So, yeah, it's been a while... okay, it's been more than a while. I'm sure my 8 followers missed me. Anyway, I recently had the opportunity to learn a whole mess of new stuff, and I wanted to start writing some of it down. I had a 3 week fling with Ruby, followed by a longer affair with JavaScript. I was a static language guy for the last 3 years, so all this dynamic nonsense had my head spinning... and you know what?

I liked it.

I always thought of JavaScript development as painful; a sort of necessary evil. Turns out there's an obscenely powerful language there, if you go looking for it (incidentally, start by looking here). Among those powerful features are first-class functions. For example, suppose I run the following code:

var greeter = {
  name: 'Mike',
  greet: function() {
    return 'hello ' + this.name;

greeter.greet(); // -> 'hello Mike'

Nothing shocking there, right? Well, I can actually reach into the greeter object and steal the greet function:

var greet = greeter.greet;
greet(); // -> 'hello '

Okay, kind of cool, but not quite the expected result. Where did my name go? When you grab a function that references this and invoke it, you need to tell JavaScript what context you want to execute it in. The built-in JavaScript Function object has a method called apply which lets you set the value of this (it also lets you pass an argument array if you want to):

var bob = {
  name: 'Bob'

greet.apply(bob); // -> 'hello Bob'

This is called the apply invocation pattern. What can you do with it? That'll have to wait for another post...

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