Can you write nested functions in JavaScript?

Question

Is this really possible.

Yes.

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.
}
console.log(a(3)(4));

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 () {
            alert('a');
        },
        b: function () {
            alert('b');
        }
    };
    foo = funcs[a_or_b];
}

foo();
pickAFunction('a');
foo();
pickAFunction('b');
foo();

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);
   alert(returnedFunction(2));

Would alert you with 5.

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(){

   }
}

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

An alternative solution with ES6 to other answers:

const currying = (x) => (y)=> x + y;
console.log(currying(5)(3));

will print to console: 8