Can you write nested functions in JavaScript?

Can you write nested functions in JavaScript?

I am wondering if JavaScript supports writing a function within another function, or nested functions (I read it in a blog). Is this really possible?. In fact, I have used these but am unsure of this concept. I am really unclear on this — please help!


Solution 1:

Is this really possible.


function a(x) {    // <-- function
  function b(y) { // <-- inner function
    return x + y; // <-- use variables from outer scope
  return b;       // <-- you can even return a function.

Solution 2:

The following is nasty, but serves to demonstrate how you can treat functions like any other kind of object.

var foo = function () { alert('default function'); }

function pickAFunction(a_or_b) {
    var funcs = {
        a: function () {
        b: function () {
    foo = funcs[a_or_b];


Solution 3:

Functions are first class objects that can be:

  • Defined within your function
  • Created just like any other variable or object at any point in your function
  • Returned from your function (which may seem obvious after the two above, but still)

To build on the example given by Kenny:

   function a(x) {
      var w = function b(y) {
        return x + y;
      return w;

   var returnedFunction = a(3);

Would alert you with 5.

Solution 4:

Yes, it is possible to write and call a function nested in another function.

Try this:

function A(){
   B(); //call should be B();
   function B(){


Solution 5:

Not only can you return a function which you have passed into another function as a variable, you can also use it for calculation inside but defining it outside. See this example:

    function calculate(a,b,fn) {
      var c = a * 3 + b + fn(a,b);
      return  c;

    function sum(a,b) {
      return a+b;

    function product(a,b) {
      return a*b;

    document.write(calculate (10,20,sum)); //80
    document.write(calculate (10,20,product)); //250