Question

What's the output?

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();

• A: "Woof I am Mara", TypeError
• B: "Woof I am Mara", "Woof I am Mara"
• C: "Woof I am Mara", undefined
• D: TypeError, TypeError

Answer 1

Answer: A

We can delete properties from objects using the delete keyword, also on the prototype. By deleting a property on the prototype, it is not available anymore in the prototype chain. In this case, the bark function is not available anymore on the prototype after delete Dog.prototype.bark, yet we still try to access it.

When we try to invoke something that is not a function, a TypeError is thrown. In this case TypeError: pet.bark is not a function, since pet.bark is undefined.