JavaScript Interview questions

Those are just some random questions I sometimes ask when evaluating junior developers.

⭐

• What is the difference between == and === ?

• What is the output ?

  const animals = {};
  ['zebra', 'horse'].forEach(function(k) {
  	animals[k] = undefined;
  });
  console.log(animals); ?

⭐⭐

• What is the difference ?

  [1,2,3].map(/*() => { ... }*/);
  [1,2,3].forEach(/*() => { ... }*/);

• What is the output ?

  const numbersAndStuff = [0, 2, 4, 6, undefined];
  const filtered = numbersAndStuff.filter(element => element);
  console.log(filtered);

• What is a closure ?
• What is a Promise ?

• What is the output ?

  function updateUserName(user, name) {
  user.name = name;
  return user;
  }
  const john = {
  name: 'John',
  age: 48,
  };
  console.log('John', john);
  const jane = updateUserName(john, 'Jane');
  console.log('Jane', jane);
  console.log('John', john);

⭐⭐⭐

• What is the event loop ?