JavaScript Interview Questions You Will Actually Get Asked

Q: What is a closure?

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

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

const counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3

The inner function "closes over" the count variable. Each call to makeCounter() creates a new, independent counter.

Q: What is the output of this code?

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 100);
}
// Output: 3, 3, 3 (not 0, 1, 2)

Because var is function-scoped, all three callbacks share the same i, which is 3 by the time the timeouts fire. Fix it with let (block-scoped) instead of var.

Q: What is the event loop?

JavaScript runs on a single thread. The event loop is the mechanism that handles asynchronous operations. When you call setTimeout or fetch, the operation is handed off to the browser or Node.js runtime. When the operation completes, its callback is placed in a queue. The event loop picks callbacks from the queue and runs them on the main thread when the call stack is empty.

Q: What is the output of this code?

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");

// Output: 1, 4, 3, 2

Synchronous code runs first (1, 4). Microtasks (promises) run before macrotasks (setTimeout), so 3 prints before 2, even though both have a delay of 0.

Q: What is a Promise?

A Promise represents a value that may not be available yet. It is in one of three states: pending, fulfilled, or rejected.

function fetchUser(id: number): Promise<string> {
  return new Promise((resolve, reject) => {
    if (id > 0) {
      resolve(`User ${id}`);
    } else {
      reject(new Error("Invalid ID"));
    }
  });
}

// Using .then/.catch
fetchUser(1)
  .then((user) => console.log(user))
  .catch((err) => console.error(err));

// Using async/await
async function main() {
  try {
    const user = await fetchUser(1);
    console.log(user);
  } catch (err) {
    console.error(err);
  }
}
main();

Q: When would you use Promise.all vs Promise.allSettled?

Promise.all rejects as soon as any promise rejects. Use it when all results are required. Promise.allSettled waits for all promises to complete regardless of success or failure. Use it when you want partial results.

Q: How does this work in JavaScript?

this depends on how a function is called, not where it is defined:

• In a regular function call, this is the global object (or undefined in strict mode)
• In a method call (obj.method()), this is the object
• In an arrow function, this is inherited from the enclosing scope
• With bind, call, or apply, you set this explicitly

const user = {
  name: "Wanjiku",
  greet() {
    console.log(`Hi, I am ${this.name}`);
  },
  greetLater() {
    // Arrow function inherits this from greetLater
    setTimeout(() => console.log(`Hi, I am ${this.name}`), 100);
  },
};

user.greet();      // Hi, I am Wanjiku
user.greetLater(); // Hi, I am Wanjiku

const greet = user.greet;
greet();           // Hi, I am undefined (this is global)

Q: Remove duplicates from an array.

const removeDuplicates = (arr: any[]) => [...new Set(arr)];
console.log(removeDuplicates([1, 2, 2, 3, 3, 3])); // [1, 2, 3]

Q: Debounce a function.

function debounce(fn: Function, delay: number) {
  let timer: ReturnType<typeof setTimeout>;
  return function (...args: any[]) {
    clearTimeout(timer);
    timer = setTimeout(() => fn(...args), delay);
  };
}

const handleSearch = debounce((query: string) => {
  console.log("Searching:", query);
}, 300);

Q: Deep clone an object without using JSON.parse/stringify.

function deepClone<T>(obj: T): T {
  if (obj === null || typeof obj !== "object") return obj;
  if (Array.isArray(obj)) return obj.map(deepClone) as T;
  const clone = {} as T;
  for (const key in obj) {
    if (Object.hasOwn(obj, key)) {
      (clone as any)[key] = deepClone((obj as any)[key]);
    }
  }
  return clone;
}