Can you write nested functions in JavaScript?

Question

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!

Answer 1

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));

• Yekver: This method is called currying.
• CiY3: so are functions a type of object? sorry for necro
• zcoop98: @CiY3 They are, via MDN: "In JavaScript, functions are first-class objects, because they can have properties and methods just like any other object. What distinguishes them from other objects is that functions can be called. In brief, they are Function objects."

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

• Mike Sherov: Great example. I would add that it's important to note that functions defined inside other functions only exist in that functions scope (unless, of course, you assign a global function to it, as per this example).
• Alex Lomia: Treat those functions like objects they are
• Jonathan Laliberte: why is that nasty exactly? Seems like it can be a useful alternative to a switch case

Answer 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);
 alert(returnedFunction(2));

Would alert you with 5.

• Yekver: This method is called currying.

Answer 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(){
 }
}

Answer 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

Answer 6

An alternative solution with ES6 to other answers:

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

will print to console: 8

• user10119132: A great read on this and a reason as to why it is called 'currying' can be found here javascript.info/currying-partials#currying-what-for

Answer 7

function calculate(num1) {
 // arrow function
 return (num2) => num1 + num2;
}
// Invoke the function
console.log(calculate(4)(6));

Answer 8

Is this really possible?

It is very possible. You can do anything with functions in javascript since it is a first class function programming language.

function add(x, y) {
 // we can define another function inside the add function to print our answer
 function print(ans) {
 console.log(ans)
 }
 const ans = x + y
 print(ans)
 return ans
}
add(1, 2)