JavaScript Closures

What is a closure?

A closure is created when a function “remembers” its outer scope, even after that outer function has finished executing. In other words:

• A closure is a combination of a function + the scope in which it was created.

This allows the function to access variables from its outer function scope even after the outer function has returned.

function outer() {
 let name = "Alice";

function inner() {
 console.log("Hello, " + name);
}
 return inner;
}

let greet = outer(); // outer() runs, returns inner
greet(); // Hello, Alice

• outer runs and creates the variable name.
• outer returns the inner function.
• greet now holds the inner function.
• When you call greet(), it still has access to name, even though outer has finished.

counter using closures

Closures are commonly used to remember state.

function createCounter() {
 let count = 0;

return function() {
 count++;
 console.log(count);
 };
}

const counter = createCounter();

counter(); // 1
counter(); // 2
counter(); // 3

• count lives inside createCounter, but counter() still remembers it because of the closure.

Why closures are useful?

Data privacy:

Keep variables “private” and only accessible through functions.

function secretNumber() {
 let secret = 42;

return {
 getSecret: function() {
return secret;
 }
  };
}

const obj = secretNumber();
console.log(obj.getSecret()); // 42
console.log(obj.secret);      // undefined