Closures

10 min read

What Is a Closure?

A closure is a function that remembers the variables from its surrounding scope even after that scope has finished executing. Every function in JavaScript forms a closure over its lexical environment.

javascript

function makeCounter(start = 0) {
  let count = start;
  return {
    increment() { count++; },
    decrement() { count--; },
    value()     { return count; }
  };
}

const counter = makeCounter(10);
counter.increment();
counter.increment();
counter.decrement();
console.log(counter.value()); // 11
// `count` is private — not accessible from outside

Closures enable data privacy, factory functions, memoization, and partial application. They are one of JavaScript's most powerful — and most misunderstood — features.

Practical Example: Memoization

javascript

function memoize(fn) {
  const cache = new Map();
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

const slowSquare = (n) => {
  // imagine expensive computation
  return n * n;
};
const fastSquare = memoize(slowSquare);
fastSquare(42); // computed
fastSquare(42); // returned from cache

Closures hold references to outer variables, preventing garbage collection. In long-running apps, storing large objects in a closure that outlives its use can cause memory leaks. Nullify unneeded references when you are done with them.

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