Why would you create a variable with value this

Question

I've seen this done alot in JavaScript and I do remember finding out why but I can't remember the answer.

I'm guessing it's something to do with scope and a function being called outside the "class" but why would one do this (preferably outlining an example):

function myClass ()
{
    var self = this;

    //...

    this.myArray = [];

    this.myFunc = function () { alert(self.myArray.length); };
}

Answer 1

In order to latch onto the variable as part of a closure.

For example:

MyClass.prototype.doStuff = function(){
  this.foundItems = [];
  var self = this;
  this.myString.replace(/.../,function(){
    // `this` is actually the `window` inside this callback
    // so we need to use `self` to invoke another method on our instance object
    self.foundItems.push( self.doOtherStuff() );
  });
};

The specific example you wrote does not need a closure if you invoke the method in the expected way:

function Foo(){
  this.array = [];
  this.myFunc = function(){
    return this.array;
  }
}
var foo = new Foo;
foo.myFunc(); // []

However, it's possible to 'break' it like so:

var f2 = foo.myFunc;
f2(); // undefined, since `this` was the window

Your code using the closure, on the other hand, is safe against this sort of tomfoolery.

Answer 2

"this" refers to the current "object". The power resides, when talking about javascript, into the fact that the "this" expression is evaluated context-wise, so for example if a method is executed in another object context, the "this" value will change respectively.