Closures

A closure is a function that remembers the variables from its outer (lexical) scope, even after that scope has finished executing.

Closures are created every time a function is defined inside another function. They're the mechanism behind private state, memoization and module patterns.

function makeCounter() {
  let count = 0; // private — only accessible via the closure
  return function increment() {
    count += 1;
    return count;
  };
}

const counter = makeCounter();
counter(); // 1
counter(); // 2
// 'count' can't be accessed directly — only through counter()

Each call to makeCounter() creates a fresh, independent closure
Used for private state, currying, memoization and event handler factories
Watch out: closures inside loops can capture the wrong variable with var

Tip

Draw or describe the counter example — it's the single most common closures interview question.