JavaScript Questions

I post multiple choice JavaScript questions on my Instagram stories, which I'll also post here! Last updated: June 12th

From basic to advanced: test how well you know JavaScript, refresh your knowledge a bit, or prepare for your coding interview! I update this repo regularly with new questions. I added the answers in the collapsed sections below the questions, simply click on them to expand it. It's just for fun, good luck!

Feel free to reach out to me!
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function sayHi() {
  console.log(name);
  console.log(age);
  var name = 'Lydia';
  let age = 21;
}

sayHi();

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 1);
}

const shape = {
  radius: 10,
  diameter() {
    return this.radius * 2;
  },
  perimeter: () => 2 * Math.PI * this.radius,
};

console.log(shape.diameter());
console.log(shape.perimeter());

+true;
!'Lydia';

const bird = {
  size: 'small',
};

const mouse = {
  name: 'Mickey',
  small: true,
};

let c = { greeting: 'Hey!' };
let d;

d = c;
c.greeting = 'Hello';
console.log(d.greeting);

let a = 3;
let b = new Number(3);
let c = 3;

console.log(a == b);
console.log(a === b);
console.log(b === c);

class Chameleon {
  static colorChange(newColor) {
    this.newColor = newColor;
    return this.newColor;
  }

  constructor({ newColor = 'green' } = {}) {
    this.newColor = newColor;
  }
}

const freddie = new Chameleon({ newColor: 'purple' });
console.log(freddie.colorChange('orange'));

let greeting;
greetign = {}; // Typo!
console.log(greetign);

function bark() {
  console.log('Woof!');
}

bark.animal = 'dog';

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const member = new Person('Lydia', 'Hallie');
Person.getFullName = function() {
  return `${this.firstName} ${this.lastName}`;
};

console.log(member.getFullName());

function Person(firstName, lastName) {
  this.firstName = firstName;
  this.lastName = lastName;
}

const lydia = new Person('Lydia', 'Hallie');
const sarah = Person('Sarah', 'Smith');

console.log(lydia);
console.log(sarah);

function sum(a, b) {
  return a + b;
}

sum(1, '2');

let number = 0;
console.log(number++);
console.log(++number);
console.log(number);

function getPersonInfo(one, two, three) {
  console.log(one);
  console.log(two);
  console.log(three);
}

const person = 'Lydia';
const age = 21;

getPersonInfo`${person} is ${age} years old`;

function checkAge(data) {
  if (data === { age: 18 }) {
    console.log('You are an adult!');
  } else if (data == { age: 18 }) {
    console.log('You are still an adult.');
  } else {
    console.log(`Hmm.. You don't have an age I guess`);
  }
}

checkAge({ age: 18 });

function getAge(...args) {
  console.log(typeof args);
}

getAge(21);

function getAge() {
  'use strict';
  age = 21;
  console.log(age);
}

getAge();

const sum = eval('10*10+5');

sessionStorage.setItem('cool_secret', 123);

var num = 8;
var num = 10;

console.log(num);

const obj = { 1: 'a', 2: 'b', 3: 'c' };
const set = new Set([1, 2, 3, 4, 5]);

obj.hasOwnProperty('1');
obj.hasOwnProperty(1);
set.has('1');
set.has(1);

const obj = { a: 'one', b: 'two', a: 'three' };
console.log(obj);

for (let i = 1; i < 5; i++) {
  if (i === 3) continue;
  console.log(i);
}

String.prototype.giveLydiaPizza = () => {
  return 'Just give Lydia pizza already!';
};

const name = 'Lydia';

name.giveLydiaPizza();

const a = {};
const b = { key: 'b' };
const c = { key: 'c' };

a[b] = 123;
a[c] = 456;

console.log(a[b]);

const foo = () => console.log('First');
const bar = () => setTimeout(() => console.log('Second'));
const baz = () => console.log('Third');

bar();
foo();
baz();

<div onclick="console.log('first div')">
  <div onclick="console.log('second div')">
    <button onclick="console.log('button')">
      Click!
    </button>
  </div>
</div>

<div onclick="console.log('div')">
  <p onclick="console.log('p')">
    Click here!
  </p>
</div>

const person = { name: 'Lydia' };

function sayHi(age) {
  return `${this.name} is ${age}`;
}

console.log(sayHi.call(person, 21));
console.log(sayHi.bind(person, 21));

function sayHi() {
  return (() => 0)();
}

console.log(typeof sayHi());

0;
new Number(0);
('');
(' ');
new Boolean(false);
undefined;

console.log(typeof typeof 1);

const numbers = [1, 2, 3];
numbers[10] = 11;
console.log(numbers);

(() => {
  let x, y;
  try {
    throw new Error();
  } catch (x) {
    (x = 1), (y = 2);
    console.log(x);
  }
  console.log(x);
  console.log(y);
})();

[[0, 1], [2, 3]].reduce(
  (acc, cur) => {
    return acc.concat(cur);
  },
  [1, 2],
);

!!null;
!!'';
!!1;

setInterval(() => console.log('Hi'), 1000);

[...'Lydia'];

function* generator(i) {
  yield i;
  yield i * 2;
}

const gen = generator(10);

console.log(gen.next().value);
console.log(gen.next().value);

const firstPromise = new Promise((res, rej) => {
  setTimeout(res, 500, 'one');
});

const secondPromise = new Promise((res, rej) => {
  setTimeout(res, 100, 'two');
});

Promise.race([firstPromise, secondPromise]).then(res => console.log(res));

let person = { name: 'Lydia' };
const members = [person];
person = null;

console.log(members);

const person = {
  name: 'Lydia',
  age: 21,
};

for (const item in person) {
  console.log(item);
}

console.log(3 + 4 + '5');

const num = parseInt('7*6', 10);

[1, 2, 3].map(num => {
  if (typeof num === 'number') return;
  return num * 2;
});

function getInfo(member, year) {
  member.name = 'Lydia';
  year = '1998';
}

const person = { name: 'Sarah' };
const birthYear = '1997';

getInfo(person, birthYear);

console.log(person, birthYear);

function greeting() {
  throw 'Hello world!';
}

function sayHi() {
  try {
    const data = greeting();
    console.log('It worked!', data);
  } catch (e) {
    console.log('Oh no an error:', e);
  }
}

sayHi();

function Car() {
  this.make = 'Lamborghini';
  return { make: 'Maserati' };
}

const myCar = new Car();
console.log(myCar.make);

(() => {
  let x = (y = 10);
})();

console.log(typeof x);
console.log(typeof y);

class Dog {
  constructor(name) {
    this.name = name;
  }
}

Dog.prototype.bark = function() {
  console.log(`Woof I am ${this.name}`);
};

const pet = new Dog('Mara');

pet.bark();

delete Dog.prototype.bark;

pet.bark();

const set = new Set([1, 1, 2, 3, 4]);

console.log(set);

// counter.js
let counter = 10;
export default counter;

// index.js
import myCounter from './counter';

myCounter += 1;

console.log(myCounter);

const name = 'Lydia';
age = 21;

console.log(delete name);
console.log(delete age);

const numbers = [1, 2, 3, 4, 5];
const [y] = numbers;

console.log(y);

const user = { name: 'Lydia', age: 21 };
const admin = { admin: true, ...user };

console.log(admin);

const person = { name: 'Lydia' };

Object.defineProperty(person, 'age', { value: 21 });

console.log(person);
console.log(Object.keys(person));

const settings = {
  username: 'lydiahallie',
  level: 19,
  health: 90,
};

const data = JSON.stringify(settings, ['level', 'health']);
console.log(data);

let num = 10;

const increaseNumber = () => num++;
const increasePassedNumber = number => number++;

const num1 = increaseNumber();
const num2 = increasePassedNumber(num1);

console.log(num1);
console.log(num2);

const value = { number: 10 };

const multiply = (x = { ...value }) => {
  console.log((x.number *= 2));
};

multiply();
multiply();
multiply(value);
multiply(value);

[1, 2, 3, 4].reduce((x, y) => console.log(x, y));

class Dog {
  constructor(name) {
    this.name = name;
  }
};

class Labrador extends Dog {
  // 1
  constructor(name, size) {
    this.size = size;
  }
  // 2
  constructor(name, size) {
    super(name);
    this.size = size;
  }
  // 3
  constructor(size) {
    super(name);
    this.size = size;
  }
  // 4
  constructor(name, size) {
    this.name = name;
    this.size = size;
  }

};

// index.js
console.log('running index.js');
import { sum } from './sum.js';
console.log(sum(1, 2));

// sum.js
console.log('running sum.js');
export const sum = (a, b) => a + b;

console.log(Number(2) === Number(2));
console.log(Boolean(false) === Boolean(false));
console.log(Symbol('foo') === Symbol('foo'));

const name = 'Lydia Hallie';
console.log(name.padStart(13));
console.log(name.padStart(2));

console.log('🥑' + '💻');

function* startGame() {
  const answer = yield 'Do you love JavaScript?';
  if (answer !== 'Yes') {
    return "Oh wow... Guess we're gone here";
  }
  return 'JavaScript loves you back ❤️';
}

const game = startGame();
console.log(/* 1 */); // Do you love JavaScript?
console.log(/* 2 */); // JavaScript loves you back ❤️

console.log(String.raw`Hello\nworld`);

async function getData() {
  return await Promise.resolve('I made it!');
}

const data = getData();
console.log(data);

function addToList(item, list) {
  return list.push(item);
}

const result = addToList('apple', ['banana']);
console.log(result);

const box = { x: 10, y: 20 };

Object.freeze(box);

const shape = box;
shape.x = 100;

console.log(shape);

const { name: myName } = { name: 'Lydia' };

console.log(name);

function sum(a, b) {
  return a + b;
}

const add = () => {
  const cache = {};
  return num => {
    if (num in cache) {
      return `From cache! ${cache[num]}`;
    } else {
      const result = num + 10;
      cache[num] = result;
      return `Calculated! ${result}`;
    }
  };
};

const addFunction = add();
console.log(addFunction(10));
console.log(addFunction(10));
console.log(addFunction(5 * 2));

const myLifeSummedUp = ['☕', '💻', '🍷', '🍫'];

for (let item in myLifeSummedUp) {
  console.log(item);
}

for (let item of myLifeSummedUp) {
  console.log(item);
}

const list = [1 + 2, 1 * 2, 1 / 2];
console.log(list);

function sayHi(name) {
  return `Hi there, ${name}`;
}

console.log(sayHi());

var status = '😎';

setTimeout(() => {
  const status = '😍';

  const data = {
    status: '🥑',
    getStatus() {
      return this.status;
    },
  };

  console.log(data.getStatus());
  console.log(data.getStatus.call(this));
}, 0);

const person = {
  name: 'Lydia',
  age: 21,
};

let city = person.city;
city = 'Amsterdam';

console.log(person);

function checkAge(age) {
  if (age < 18) {
    const message = "Sorry, you're too young.";
  } else {
    const message = "Yay! You're old enough!";
  }

  return message;
}

console.log(checkAge(21));

fetch('https://www.website.com/api/user/1')
  .then(res => res.json())
  .then(res => console.log(res));

function getName(name) {
  const hasName = //
}

console.log('I want pizza'[0]);

function sum(num1, num2 = num1) {
  console.log(num1 + num2);
}

sum(10);

// module.js
export default () => 'Hello world';
export const name = 'Lydia';

// index.js
import * as data from './module';

console.log(data);

class Person {
  constructor(name) {
    this.name = name;
  }
}

const member = new Person('John');
console.log(typeof member);

let newList = [1, 2, 3].push(4);

console.log(newList.push(5));

function giveLydiaPizza() {
  return 'Here is pizza!';
}

const giveLydiaChocolate = () =>
  "Here's chocolate... now go hit the gym already.";

console.log(giveLydiaPizza.prototype);
console.log(giveLydiaChocolate.prototype);

const person = {
  name: 'Lydia',
  age: 21,
};

for (const [x, y] of Object.entries(person)) {
  console.log(x, y);
}

function getItems(fruitList, ...args, favoriteFruit) {
  return [...fruitList, ...args, favoriteFruit]
}

getItems(["banana", "apple"], "pear", "orange")

function nums(a, b) {
  if (a > b) console.log('a is bigger');
  else console.log('b is bigger');
  return
  a + b;
}

console.log(nums(4, 2));
console.log(nums(1, 2));

class Person {
  constructor() {
    this.name = 'Lydia';
  }
}

Person = class AnotherPerson {
  constructor() {
    this.name = 'Sarah';
  }
};

const member = new Person();
console.log(member.name);

const info = {
  [Symbol('a')]: 'b',
};

console.log(info);
console.log(Object.keys(info));

const getList = ([x, ...y]) => [x, y]
const getUser = user => { name: user.name, age: user.age }

const list = [1, 2, 3, 4]
const user = { name: "Lydia", age: 21 }

console.log(getList(list))
console.log(getUser(user))

const name = 'Lydia';

console.log(name());

// 🎉✨ This is my 100th question! ✨🎉

const output = `${[] && 'Im'}possible!
You should${'' && `n't`} see a therapist after so much JavaScript lol`;

const one = false || {} || null;
const two = null || false || '';
const three = [] || 0 || true;

console.log(one, two, three);

const myPromise = () => Promise.resolve('I have resolved!');

function firstFunction() {
  myPromise().then(res => console.log(res));
  console.log('second');
}

async function secondFunction() {
  console.log(await myPromise());
  console.log('second');
}

firstFunction();
secondFunction();

const set = new Set();

set.add(1);
set.add('Lydia');
set.add({ name: 'Lydia' });

for (let item of set) {
  console.log(item + 2);
}

Promise.resolve(5);

function compareMembers(person1, person2 = person) {
  if (person1 !== person2) {
    console.log('Not the same!');
  } else {
    console.log('They are the same!');
  }
}

const person = { name: 'Lydia' };

compareMembers(person);

const colorConfig = {
  red: true,
  blue: false,
  green: true,
  black: true,
  yellow: false,
};

const colors = ['pink', 'red', 'blue'];

console.log(colorConfig.colors[1]);

console.log('❤️' === '❤️');

const emojis = ['✨', '🥑', '😍'];

emojis.map(x => x + '✨');
emojis.filter(x => x !== '🥑');
emojis.find(x => x !== '🥑');
emojis.reduce((acc, cur) => acc + '✨');
emojis.slice(1, 2, '✨');
emojis.splice(1, 2, '✨');

const food = ['🍕', '🍫', '🥑', '🍔'];
const info = { favoriteFood: food[0] };

info.favoriteFood = '🍝';

console.log(food);

JSON.parse();

let name = 'Lydia';

function getName() {
  console.log(name);
  let name = 'Sarah';
}

getName();

function* generatorOne() {
  yield ['a', 'b', 'c'];
}

function* generatorTwo() {
  yield* ['a', 'b', 'c'];
}

const one = generatorOne();
const two = generatorTwo();

console.log(one.next().value);
console.log(two.next().value);

console.log(`${(x => x)('I love')} to program`);

let config = {
  alert: setInterval(() => {
    console.log('Alert!');
  }, 1000),
};

config = null;

const myMap = new Map();
const myFunc = () => 'greeting';

myMap.set(myFunc, 'Hello world!');

//1
myMap.get('greeting');
//2
myMap.get(myFunc);
//3
myMap.get(() => 'greeting');

const person = {
  name: 'Lydia',
  age: 21,
};

const changeAge = (x = { ...person }) => (x.age += 1);
const changeAgeAndName = (x = { ...person }) => {
  x.age += 1;
  x.name = 'Sarah';
};

changeAge(person);
changeAgeAndName();

console.log(person);

function sumValues(x, y, z) {
  return x + y + z;
}

let num = 1;
const list = ['🥳', '🤠', '🥰', '🤪'];

console.log(list[(num += 1)]);

const person = {
  firstName: 'Lydia',
  lastName: 'Hallie',
  pet: {
    name: 'Mara',
    breed: 'Dutch Tulip Hound',
  },
  getFullName() {
    return `${this.firstName} ${this.lastName}`;
  },
};

console.log(person.pet?.name);
console.log(person.pet?.family?.name);
console.log(person.getFullName?.());
console.log(member.getLastName?.());

const groceries = ['banana', 'apple', 'peanuts'];

if (groceries.indexOf('banana')) {
  console.log('We have to buy bananas!');
} else {
  console.log(`We don't have to buy bananas!`);
}

const config = {
  languages: [],
  set language(lang) {
    return this.languages.push(lang);
  },
};

console.log(config.language);

const name = 'Lydia Hallie';

console.log(!typeof name === 'object');
console.log(!typeof name === 'string');

const add = x => y => z => {
  console.log(x, y, z);
  return x + y + z;
};

add(4)(5)(6);

async function* range(start, end) {
  for (let i = start; i <= end; i++) {
    yield Promise.resolve(i);
  }
}

(async () => {
  const gen = range(1, 3);
  for await (const item of gen) {
    console.log(item);
  }
})();

const myFunc = ({ x, y, z }) => {
  console.log(x, y, z);
};

myFunc(1, 2, 3);

function getFine(speed, amount) {
  const formattedSpeed = new Intl.NumberFormat('en-US', {
    style: 'unit',
    unit: 'mile-per-hour'
  }).format(speed);

  const formattedAmount = new Intl.NumberFormat('en-US', {
    style: 'currency',
    currency: 'USD'
  }).format(amount);

  return `The driver drove ${formattedSpeed} and has to pay ${formattedAmount}`;
}

console.log(getFine(130, 300))

const spookyItems = ['👻', '🎃', '🕸'];
({ item: spookyItems[3] } = { item: '💀' });

console.log(spookyItems);

const name = 'Lydia Hallie';
const age = 21;

console.log(Number.isNaN(name));
console.log(Number.isNaN(age));

console.log(isNaN(name));
console.log(isNaN(age));

const randomValue = 21;

function getInfo() {
  console.log(typeof randomValue);
  const randomValue = 'Lydia Hallie';
}

getInfo();

const myPromise = Promise.resolve('Woah some cool data');

(async () => {
  try {
    console.log(await myPromise);
  } catch {
    throw new Error(`Oops didn't work`);
  } finally {
    console.log('Oh finally!');
  }
})();

const emojis = ['🥑', ['✨', '✨', ['🍕', '🍕']]];

console.log(emojis.flat(1));

class Counter {
  constructor() {
    this.count = 0;
  }

  increment() {
    this.count++;
  }
}

const counterOne = new Counter();
counterOne.increment();
counterOne.increment();

const counterTwo = counterOne;
counterTwo.increment();

console.log(counterOne.count);

const myPromise = Promise.resolve(Promise.resolve('Promise!'));

function funcOne() {
  myPromise.then(res => res).then(res => console.log(res));
  setTimeout(() => console.log('Timeout!', 0));
  console.log('Last line!');
}

async function funcTwo() {
  const res = await myPromise;
  console.log(await res);
  setTimeout(() => console.log('Timeout!', 0));
  console.log('Last line!');
}

funcOne();
funcTwo();

// sum.js
export default function sum(x) {
  return x + x;
}

// index.js
import * as sum from './sum';

const handler = {
  set: () => console.log('Added a new property!'),
  get: () => console.log('Accessed a property!'),
};

const person = new Proxy({}, handler);

person.name = 'Lydia';
person.name;

const person = { name: 'Lydia Hallie' };

Object.seal(person);

const person = {
  name: 'Lydia Hallie',
  address: {
    street: '100 Main St',
  },
};

Object.freeze(person);

const add = x => x + x;

function myFunc(num = 2, value = add(num)) {
  console.log(num, value);
}

myFunc();
myFunc(3);

class Counter {
  #number = 10

  increment() {
    this.#number++
  }

  getNum() {
    return this.#number
  }
}

const counter = new Counter()
counter.increment()

console.log(counter.#number)

const teams = [
  { name: 'Team 1', members: ['Paul', 'Lisa'] },
  { name: 'Team 2', members: ['Laura', 'Tim'] },
];

function* getMembers(members) {
  for (let i = 0; i < members.length; i++) {
    yield members[i];
  }
}

function* getTeams(teams) {
  for (let i = 0; i < teams.length; i++) {
    // ✨ SOMETHING IS MISSING HERE ✨
  }
}

const obj = getTeams(teams);
obj.next(); // { value: "Paul", done: false }
obj.next(); // { value: "Lisa", done: false }

const person = {
  name: 'Lydia Hallie',
  hobbies: ['coding'],
};

function addHobby(hobby, hobbies = person.hobbies) {
  hobbies.push(hobby);
  return hobbies;
}

addHobby('running', []);
addHobby('dancing');
addHobby('baking', person.hobbies);

console.log(person.hobbies);

class Bird {
  constructor() {
    console.log("I'm a bird. 🦢");
  }
}

class Flamingo extends Bird {
  constructor() {
    console.log("I'm pink. 🌸");
    super();
  }
}

const pet = new Flamingo();

const emojis = ['🎄', '🎅🏼', '🎁', '⭐'];

/* 1 */ emojis.push('🦌');
/* 2 */ emojis.splice(0, 2);
/* 3 */ emojis = [...emojis, '🥂'];
/* 4 */ emojis.length = 0;

const person = {
  name: "Lydia Hallie",
  age: 21
}

[...person] // ["Lydia Hallie", 21]

let count = 0;
const nums = [0, 1, 2, 3];

nums.forEach(num => {
	if (num) count += 1
})

console.log(count)

function getFruit(fruits) {
	console.log(fruits?.[1]?.[1])
}

getFruit([['🍊', '🍌'], ['🍍']])
getFruit()
getFruit([['🍍'], ['🍊', '🍌']])

class Calc {
	constructor() {
		this.count = 0 
	}

	increase() {
		this.count ++
	}
}

const calc = new Calc()
new Calc().increase()

console.log(calc.count)

const user = {
	email: "[email protected]",
	password: "12345"
}

const updateUser = ({ email, password }) => {
	if (email) {
		Object.assign(user, { email })
	}

	if (password) {
		user.password = password
	}

	return user
}

const updatedUser = updateUser({ email: "[email protected]" })

console.log(updatedUser === user)

const fruit = ['🍌', '🍊', '🍎']

fruit.slice(0, 1)
fruit.splice(0, 1)
fruit.unshift('🍇')

console.log(fruit)

const animals = {};
let dog = { emoji: '🐶' }
let cat = { emoji: '🐈' }

animals[dog] = { ...dog, name: "Mara" }
animals[cat] = { ...cat, name: "Sara" }

console.log(animals[dog])

const user = {
	email: "[email protected]",
	updateEmail: email => {
		this.email = email
	}
}

user.updateEmail("[email protected]")
console.log(user.email)

const promise1 = Promise.resolve('First')
const promise2 = Promise.resolve('Second')
const promise3 = Promise.reject('Third')
const promise4 = Promise.resolve('Fourth')

const runPromises = async () => {
	const res1 = await Promise.all([promise1, promise2])
	const res2  = await Promise.all([promise3, promise4])
	return [res1, res2]
}

runPromises()
	.then(res => console.log(res))
	.catch(err => console.log(err))

const keys = ["name", "age"]
const values = ["Lydia", 22]

const method = /* ?? */
Object[method](keys.map((_, i) => {
	return [keys[i], values[i]]
})) // { name: "Lydia", age: 22 }

const createMember = ({ email, address = {}}) => {
	const validEmail = /.+\@.+\..+/.test(email)
	if (!validEmail) throw new Error("Valid email pls")

	return {
		email,
		address: address ? address : null
	}
}

const member = createMember({ email: "[email protected]" })
console.log(member)

let randomValue = { name: "Lydia" }
randomValue = 23

if (!typeof randomValue === "string") {
	console.log("It's not a string!")
} else {
	console.log("Yay it's a string!")
}