Prototype and Scriptaculous: Taking the Pain out of JavaScript

2.3. Closures / Anonymous Functions and Functions as Objects

In JavaScript, the definition:

function add(x,y)
{
      return x+y;
}

is equivalent to:

var add = function(x,y)
{
      return x+y;
}

Both create a variable named add to reference a function object. In the second version, however, the function is anonymous. The ability to create anonymous functions means that we can create specialized functions (or closures).

var createAdditionClosure = function(x,y)
{
      return function() {x+y;}
}

And then call this via:

var fn = createAdditionClosure(1,2);
var value = fn();

This ability allows us to capture contextual state and wrap it up into an event handler:

element.onClick = function() { // operations on current context }