Question

I was reading a post about how to fire a function after a window resize was complete and came across some examples that assigned self executing anonymous functions to variables:

var delay = (function(){
    var timer = 0;
    return function(callback, ms){
        clearTimeout (timer);
        timer = setTimeout(callback, ms);
    };
})();

$(window).resize(function() {
    delay(function(){
      alert('Resize...');
      //...
    }, 500);
});

What is the difference/benefit of making the function operand self executing as opposed to it’s traditional use? i.e.

var delay = function() { ... 

Answer 1

The main reason for this is namespacing variables. Functions introduce a new variable scope. In the case of the above example, timer is not clobbering the global namespace while still being available to the code that needs it.

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Since I apparently need to clarify:

The goal is to have a variable outside the function:

var timer;

function delay() {
   // use timer
}

Because if the variable would be inside the function, it would be reinitialized every time. We want a persistent value outside the function though.

In the above code timer is a global variable though. We don’t want that. To avoid that, we close the variable up in a new scope so it’s accessible to the delay function, but not globally:

var delay = (function () {
    var timer;

    return function () {
        // use timer
    };
})();

delay is now a function just as before which can use timer outside itself, yet timer is not in the global scope.