Closure demystified. Closure is best expressed when a function returns an anonymous function. If the nested anonymous function references a variable within the parent function context, that variable reference will be accessible when the nested function is invoked.

js-closure

var multiplyByNumber = function (x) {
    return function(y) {
        // this nested anonymous function references 
        // parent 'x' arg variable
        return x * y;
    }
}

/**
 * multiplyBy5 below is a function because multiplyByNumber returns a function.
 * To reiterate, the below statement is the equivalent of
 * var multiplyBy5 = function(y) {
 *                      return 5 * y;
 *                   }
 */
var multiplyBy5 = multiplyByNumber(5); 

var multiply10x5 = multiplyBy5(10);
console.log(multiply10x5);    // logs 50

/**
 * So we see closure in action.   multiplyBy5 points to the nested anonymous
 * return function within multiplyByNumber and
 * that nested function was initially given an 'x' value of 5 on line 16.
 * This accessible 'x' value from the multipylyByNumber function context is available
 * because of closure.  When this child function is invoked it still
 * has access to the parent's function scope.
 */