An inner function always has access to the variables and params of its outer function, even after the outer function has been returned.

closure.js

//@see: http://davidshariff.com/blog/javascript-scope-chain-and-closures/
function foo() {
    var a = 'private variable';
    return function bar() {
        alert(a);
    }
}

var callAlert = foo();

callAlert(); // private variable