Purpose of passing global vars to self-invoking function or "IIFE"

Question

I see a lot of this in older JavaScript

(function (w){

    w.bar = 'baz';

 })(window); 

What is the advantage of the above, over

(function(){

    window.bar = 'baz';

})(); 

same goes for any global variable, or variable defined outside the IIFE.

Answer 1

In practice there is not much (any?) difference wit the example you have given, but you have probably oversimplified it from the code you are actually looking at.

In a more realistic program, you will have scopes and calls backs which are triggered and run in the eventloop asynchronously, and you are binding the variable to a specific instance in a closure -- so;

(function (w){
    setTimeout(function(){w.bar = 'baz';},100);
})(window); 
window = window2;

The bar is still being set in the original window, since that is what is bound to w - where in

(function (){
    setTimeout(function(){window.bar = 'baz';},10);
})(window); 
window = window2;

It will be set in the instance window2, since that is how the window is bound when the execution of the code eventually happens.

In this example "window" is a global variable, but the same is the case regardless of the scope of the variable that is bound.

Answer 2

1. Makes it explicit that you are using (and probably modifying) globals in the function.
2. Allows you to modify the behavior in the future. Maybe you have a mockWindow for a unit test. Maybe you are using Node.js and you don't have a window, but want to add to the globals var instead.

p.s. IMO the trivial performance gain mentioned by @Rayon is a red herring.

Answer 3

Passing window is done because local variables are easier and faster to access than global variables ...this may show slight difference in performance . The method really comes in handy with module patterns and/or dependency injection .

Example

var moduleFirst = (function(){
    var name = "harry";
    return {
        firstparam : name
    }
})();
var moduleTwo = (function(x){
    console.log(x.firstparam);
})(moduleFirst)

Output will be : harry

So when window gets passed in IIFE ; all of its revealed methods are available in a local variable .