JavaScript Closures and the Event Loop, Explained Simply

Two of the most asked JavaScript interview topics, explained with small examples you can reproduce on a whiteboard.

Closures

A closure is a function that remembers the variables from the scope where it was created, even after that outer function has finished running.

function createCounter() {
  let count = 0;
  return () => ++count;
}

const next = createCounter();
next(); // 1
next(); // 2 — count is preserved by the closure

Closures power data privacy, function factories, memoization and many callback patterns.

The event loop

JavaScript runs on a single thread. Synchronous code runs on the call stack first. Async callbacks wait in queues: the microtask queue (promises) is emptied before the next macrotask (setTimeout, I/O) runs.

console.log('A');
setTimeout(() => console.log('B'), 0);
Promise.resolve().then(() => console.log('C'));
console.log('D');

// Output: A, D, C, B

• JavaScript
• Closures
• Event Loop