JavaScript Interview Questions Cheat Sheet: Closures, this, Hoisting & Promises

JavaScript interviews probe mental models: scope and closures, this binding, hoisting, the event loop, and promises. Framework trivia is secondary. Be ready to write a small debounce, explain ===, and say what let does differently from var.

This JavaScript cheat sheet is written for people searching for a fast, accurate reference: students, junior developers, and teams shipping production software. Pin it, copy the snippets, and come back when syntax slips.

Quick reference

• Closure: A function remembers variables from the scope where it was created, even after that scope returned.
• this: Depends on call site: method, call/apply, new, or lexical in arrows.
• Hoisting: var and function declarations hoist. let/const are in the TDZ until initialized.
• Event loop: Call stack runs to empty; then microtasks (promises); then macrotasks (timeouts, I/O).
• ===: No coercion. == coerces and is a bug factory.
• Prototype: Objects delegate via [[Prototype]]. Classes are syntax over this.
• Truthy: 0, "", null, undefined, NaN, false are falsy. [] and {} are truthy.
• Debounce: Delay a function until calls stop. Throttle limits rate. Know the difference.

Copy-paste examples

Closure counter and lexical this

The inner function closes over n. The arrow inside the method keeps this from the method.

function makeCounter(start = 0) {
  let n = start;
  return () => ++n;
}
const c = makeCounter();
console.log(c(), c()); // 1 2
const api = {
  token: 'abc',
  headers: function () {
    return () => ({ Authorization: this.token });
  },
};

Common mistakes

• Confusing this in arrows vs methods and “fixing” it with random .bind(this).
• Saying JavaScript is multi-threaded because of async.
• Mutating arguments or prototypes in interview code without discussing the cost.
• Cannot explain a promise vs a callback besides “async/await is newer.”

FAQ

1. What is hoisting in one example? console.log(a) before var a = 1 prints undefined. The same with let throws. Function declarations are available before their line.

1. How do you clone an array of objects? Shallow: [...list] or list.map. Deep: structuredClone. Mention that nested objects stay shared on shallow copy.

1. null vs undefined? undefined means not assigned. null is an intentional empty object value. typeof null is "object" — a historic bug.

Related JavaScript cheat sheets

• Javascript Cheat Sheet 2026
• Javascript Array Methods Cheat Sheet
• Javascript String Methods Cheat Sheet

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