JavaScript Nested Functions

All functions have access to the global scope.

In fact, in JavaScript, all functions have access to the scope "above" them.

JavaScript supports nested functions. Nested functions have access to the scope "above" them.

In this example, the inner function plus() has access to the counter variable in the parent function:

function add() {
  let counter = 0;
  function plus() {counter += 1;}
  plus();
  return counter;
}

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This could have solved the counter dilemma, if we could reach the plus() function from the outside.

We also need to find a way to execute counter = 0 only once.

We need a closure.

JavaScript Closures

Remember self-invoking functions? What does this function do?

const add = (function () {
let counter = 0;
return function () {counter += 1; return counter}
})();

add();
add();
add();

// the counter is now 3

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The variable add is assigned to the return value of a self-invoking function.

The self-invoking function only runs once. It sets the counter to zero (0), and returns a function expression.

This way add becomes a function. The "wonderful" part is that it can access the counter in the parent scope.

This is called a JavaScript closure. It makes it possible for a function to have "private" variables.

The counter is protected by the scope of the anonymous function, and can only be changed using the add function.

A closure is a function having access to the parent scope, even after the parent function has closed.

Practice Excercise Practice now