What are the pro's and con's of using Function Statement v.s. Function Expression?

Hi Oly Su

First let's make sure we're talking about the same thing. A function declaration (also sometimes called a function statement) looks like this:

function myFunction() {

}

A function expression looks like this:

var myFunction = function () {

};

The main difference between the two is that a function declaration can be placed ANYWHERE in a script including at the bottom of the script. The browser's JavaScript interpreter reads the script before running anything and commits the function to memory. Because of this it can be called anywhere in the script -- even BEFORE the declaration actually appears in the script. For example, this works:

sayHi();
function sayHi() {
  alert('Hi');
};

A function expression, on the other hand, is only created during runtime -- in other words you can't call the function until the JavaScript interpreter evaluates (or runs) the expression. For example this WON'T work:

sayHi(); // produces an "undefined is not a function" error
var sayHi = function() {
  alert('Hi');
};

To avoid all this confusion, it's often a good idea to declare your functions at the beginning of a script when possible.

I do not believe there is any (or at least any significant) performance boost for using a function expression,