What is the purpose of wrapping whole Javascript files in anonymous functions like “(function(){ … })()”?

I have been reading a lot of Javascript lately and I have been noticing that the whole file is wrapped like the following in the .js files to be imported.

(function() {
    ... 
    code
    ...
})();

What is the reason for doing this rather than a simple set of constructor functions?

It's usually to namespace (see later) and control the visibility of member functions and/or variables. Think of it like an object definition. The technical name for it is an Immediately Invoked Function Expression (IIFE). jQuery plugins are usually written like this.

In Javascript, you can nest functions. So, the following is legal:

function outerFunction() {
   function innerFunction() {
      // code
   }
}

Now you can call outerFunction(), but the visiblity of innerFunction() is limited to the scope of outerFunction(), meaning it is private to outerFunction(). It basically follows the same principle as variables in Javascript:

var globalVariable;

function someFunction() {
   var localVariable;
}

Correspondingly:

function globalFunction() {
  
   var localFunction1 = function() {
       //I'm anonymous! But localFunction1 is a reference to me!
   };

   function localFunction2() {
      //I'm named!
   }
}

In the above scenario, you can call globalFunction() from anywhere, but you cannot call localFunction1 or localFunction2.

What you're doing when you write (function() { ... })(), is you're making the code inside the first set of parentheses a function literal (meaning the whole "object" is actually a function). After that, you're self-invoking the function (the final ()) that you just defined. So the major advantage of this as I mentioned before, is that you can have private methods/functions and properties:

(function() {
   var private_var;

   function private_function() {
     //code
   }
})();

In the first example, you would explicitly invoke globalFunction by name to run it. That is, you would just do globalFunction() to run it. But in the above example, you're not just defining a function; you're defining and invoking it in one go. This means that when your JavaScript file is loaded, it is immediately executed. Of course, you could do:

function globalFunction() {
    // code
}
globalFunction();

The behavior would largely be the same except for one significant difference: you avoid polluting the global scope when you use an IIFE (as a consequence it also means that you cannot invoke the function multiple times since it doesn't have a name, but since this function is only meant to be executed once, it really isn't an issue).

The neat thing with IIFEs is that you can also define things inside and only expose the parts that you want to the outside world so (an example of namespacing so you can basically create your own library/plugin):

var myPlugin = (function() {
 var private_var;

 function private_function() {
 }

 return {
    public_function1: function() {
    },
    public_function2: function() {
    }
 }
})()

Now you can call myPlugin.public_function1(), but you cannot access private_function()! So pretty similar to a class definition. To understand this better, I recommend the following links for some further reading:

• Namespacing your Javascript
• Private members in Javascript (by Douglas Crockford)

EDIT

I forgot to mention. In that final (), you can pass anything you want inside. For example, when you create jQuery plugins, you pass in jQuery or $ like so:

(function(jQ) { ... code ... })(jQuery) 

So what you're doing here is defining a function that takes in one parameter (called jQ, a local variable, and known only to that function). Then you're self-invoking the function and passing in a parameter (also called jQuery, but this one is from the outside world and a reference to the actual jQuery itself). There is no pressing need to do this, but there are some advantages:

• You can redefine a global parameter and give it a name that makes sense in the local scope.
• There is a slight performance advantage since it is faster to look things up in the local scope instead of having to walk up the scope chain into the global scope.
• There are benefits for compression (minification).

Earlier I described how these functions run automatically at startup, but if they run automatically who is passing in the arguments? This technique assumes that all the parameters you need are already defined as global variables. So if jQuery wasn't already defined as a global variable this example would not work. As you might guess, one thing jquery.js does during its initialization is define a 'jQuery' global variable, as well as its more famous '$' global variable, which allows this code to work after jQuery has been included.