Javascript Self Invoking Functions

In this article, I will discuss self-executing or self-invoking functions of javascript and their hidden power with real-world example. We will also see why using setInterval is bad in some situations and should be avoided. Let’s explore little used but extremely powerful self-invoking functions of javascript.

Self-Invoking Anonymous Function

A self-invoking anonymous runs automatically/immediately when you create it and has no name, hence called anonymous. Here is the format of self-executing anonymous function:

 // some code…

You must be aware of the fact that javascript functions run immediately when you put () after their names eg:

doStuff(); // this will run immediately


doStuff; // this will not run immediately and can be used as callback

Now because () exist at the end of self-executing anonymous function above, it will run immediately.

Interestingly, if you look at the source code of jQuery, you will see that everything is wrapped in between:

(function( window, undefined ) {
 // jQuery code

That is as can be seen also a self-executing anonymous function with arguments. A window (and undefined) argument is created and is mapped with global window object at the bottom (window).

Notice that you can also write self-executing anonymous function like this:

 // some code…

Using extra braces like before function keyword is simply coding convention and is used even by famous javascript libraries such as jQuery and also recommended by Douglas Crockford. If you are confused about that extra pair of braces, here is another easy to remember notation:

! function(){
// some code…

Notice the addition of ! before function keyword, it is essentially same in functionality to previous notation and also recommended (used by twitter too), quoting docs, section 12.4:

An ExpressionStatement cannot start with the function keyword because that might make it ambiguous with a FunctionDeclaration.

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