What is the purpose of a self executing function in javascript?

It's all about variable scoping. Variables declared in the self executing function are, by default, only available to code within the self executing function. This allows code to be written without concern of how variables are named in other blocks of JavaScript code.

For example, as mentioned in a comment by Alexander:

(function() {
  var foo = 3;
  console.log(foo);
})();

console.log(foo);

This will first log 3 and then throw an error on the next console.log because foo is not defined.

Namespacing. JavaScript's scopes are function-level.

I can't believe none of the answers mention implied globals.

The (function(){})() construct does not protect against implied globals, which to me is the bigger concern, see http://yuiblog.com/blog/2006/06/01/global-domination/

Basically the function block makes sure all the dependent "global vars" you defined are confined to your program, it does not protect you against defining implicit globals. JSHint or the like can provide recommendations on how to defend against this behavior.

The more concise var App = {} syntax provides a similar level of protection, and may be wrapped in the function block when on 'public' pages. (see Ember.js or SproutCore for real world examples of libraries that use this construct)

As far as private properties go, they are kind of overrated unless you are creating a public framework or library, but if you need to implement them, Douglas Crockford has some good ideas.

Scope isolation, maybe. So that the variables inside the function declaration don't pollute the outer namespace.

Of course, on half the JS implementations out there, they will anyway.

Is there a parameter and the "Bunch of code" returns a function?

var a = function(x) { return function() { document.write(x); } }(something);

Closure. The value of something gets used by the function assigned to a. something could have some varying value (for loop) and every time a has a new function.

One difference is that the variables that you declare in the function are local, so they go away when you exit the function and they don't conflict with other variables in other or same code.

Short answer is : to prevent pollution of the Global (or higher) scope.

IIFE (Immediately Invoked Function Expressions) is the best practice for writing scripts as plug-ins, add-ons, user scripts or whatever scripts are expected to work with other people's scripts. This ensures that any variable you define does not give undesired effects on other scripts.

This is the other way to write IIFE expression. I personally prefer this following method:

void function() {
  console.log('boo!');
  // expected output: "boo!"
}();

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void

From the example above it is very clear that IIFE can also affect efficiency and performance, because the function that is expected to be run only once will be executed once and then dumped into the void for good. This means that function or method declaration does not remain in memory.

Self executing function are used to manage the scope of a Variable.

The scope of a variable is the region of your program in which it is defined.

A global variable has global scope; it is defined everywhere in your JavaScript code and can be accessed from anywhere within the script, even in your functions. On the other hand, variables declared within a function are defined only within the body of the function. They are local variables, have local scope and can only be accessed within that function. Function parameters also count as local variables and are defined only within the body of the function.

As shown below, you can access the global variables variable inside your function and also note that within the body of a function, a local variable takes precedence over a global variable with the same name.

var globalvar = "globalvar"; // this var can be accessed anywhere within the script

function scope() {
    alert(globalvar);
    var localvar = "localvar"; //can only be accessed within the function scope
}

scope(); 

So basically a self executing function allows code to be written without concern of how variables are named in other blocks of javascript code.

First you must visit MDN IIFE , Now some points about this

• this is Immediately Invoked Function Expression. So when your javascript file invoked from HTML this function called immediately.
• This prevents accessing variables within the IIFE idiom as well as polluting the global scope.

Since functions in Javascript are first-class object, by defining it that way, it effectively defines a "class" much like C++ or C#.

That function can define local variables, and have functions within it. The internal functions (effectively instance methods) will have access to the local variables (effectively instance variables), but they will be isolated from the rest of the script.

Given your simple question: "In javascript, when would you want to use this:..."

I like @ken_browning and @sean_holding's answers, but here's another use-case that I don't see mentioned:

let red_tree = new Node(10);

(async function () {
    for (let i = 0; i < 1000; i++) {
        await red_tree.insert(i);
    }
})();

console.log('----->red_tree.printInOrder():', red_tree.printInOrder());

where Node.insert is some asynchronous action.

I can't just call await without the async keyword at the declaration of my function, and i don't need a named function for later use, but need to await that insert call or i need some other richer features (who knows?).

You can use this function to return values :

var Test = (function (){
        
        
        const alternative = function(){ return 'Error Get Function '},
        methods = {
            GetName: alternative,
            GetAge:alternative
            }
            
            

// If the condition is not met, the default text will be returned
// replace to  55 < 44
if( 55 > 44){



// Function one
methods.GetName = function (name) {
        
        return name;

};

// Function Two

methods.GetAge = function (age) {
        
        return age;

};





}










    return methods;
    
    
    }());
    
    
    
    
    
    
    
    // Call
   console.log( Test.GetName("Yehia") );

    console.log( Test.GetAge(66) );

Use of this methodology is for closures. Read this link for more about closures.

IIRC it allows you to create private properties and methods.