Question

I used to use the anonymous self-execute function in js:

((function(){
  //do something
})();

However I found this somewhere:

((function(){
  //do something
}).call(this);

What is the difference?

Answer 1

In the first example, this will be the global object (window in browser-land), unless you’re in ES5 mode, in which it’ll be undefined.

In the second example, this depends on what this was in the calling context- either global (undefined in ES5), or a object instance.

function Foo() {
    var that = this;

    (function () {
       console.log(this === window);
       console.log(this === that);
       console.log(typeof this === "undefined"); 
    }());

    (function () {
       console.log(this === window);
       console.log(this === that);
       console.log(typeof this === "undefined"); 
    }).call(this);

    (function () {
       "use strict";

       console.log(this === window);
       console.log(this === that); 
       console.log(typeof this === "undefined"); 
    }());
}

new Foo();​

[Fiddle]

You’ll also find the .call() call is microscopically slower due to the extra work the engine has to do to set the this context up.