Closures in JavaScript

A closure is created when a function is defined inside another function and gains access to the outer function’s variables — even after the outer function has finished execution.

Key points:

The inner function 'remembers' the environment (variables) in which it was created.
Closures enable private variables — variables that cannot be accessed from outside the function directly.

Simple Example:

javascript1function outerFunction() {
2  let privateVariable = 'I'm private';
3
4  function innerFunction() {
5    console.log(privateVariable); // Accesses the variable from outerFunction
6  }
7
8  return innerFunction;
9}
10
11const closureFunc = outerFunction();
12closureFunc(); // Output: 'I'm private'

Even though outerFunction has finished running, closureFunc still remembers privateVariable.

Real-world use case: Private Counters

javascript1function createCounter() {
2  let count = 0;
3
4  return {
5    increment: function() {
6      count++;
7      console.log(count);
8    },
9    decrement: function() {
10      count--;
11      console.log(count);
12    }
13  };
14}
15
16const counter = createCounter();
17counter.increment(); // 1
18counter.increment(); // 2
19counter.decrement(); // 1

Here, count is private — it can only be modified using increment and decrement.