JavaScript Nested function

It is called closure.

Basically, the function defined within other function is accessible only within this function. But may be passed as a result and then this result may be called.

It is a very powerful feature. You can see more explanation here:

javascript_closures_for_dummies.html mirror on Archive.org

function x() {}

is equivalent (or very similar) to

var x = function() {}

unless I'm mistaken.

So there is nothing funny going on.

Function-instantiation is allowed inside and outside of functions. Inside those functions, just like variables, the nested functions are local and therefore cannot be obtained from the outside scope.

function foo() {
    function bar() {
        return 1;
    }
    return bar();
}

foo manipulates bar within itself. bar cannot be touched from the outer scope unless it is defined in the outer scope.

So this will not work:

function foo() {
    function bar() {
        return 1;
    }
}

bar(); // throws error: bar is not defined

Functions are another type of variable in JavaScript (with some nuances of course). Creating a function within another function changes the scope of the function in the same way it would change the scope of a variable. This is especially important for use with closures to reduce total global namespace pollution.

The functions defined within another function won't be accessible outside the function unless they have been attached to an object that is accessible outside the function:

function foo(doBar)
{
  function bar()
  {
    console.log( 'bar' );
  }

  function baz()
  {
    console.log( 'baz' );
  }

  window.baz = baz;
  if ( doBar ) bar();
}

In this example, the baz function will be available for use after the foo function has been run, as it's overridden window.baz. The bar function will not be available to any context other than scopes contained within the foo function.

as a different example:

function Fizz(qux)
{
  this.buzz = function(){
    console.log( qux );
  };
}

The Fizz function is designed as a constructor so that, when run, it assigns a buzz function to the newly created object. That is, you'd use it like this:

const obj = new Fizz();
obj.buzz();

or more concisely (if you don't need to keep the object after calling buzz):

new Fizz().buzz();