Prototypes and Prototype Chain

JavaScript inheritance is prototype-based.

Objects can use properties and methods from another object through the prototype chain.

Simple example

const user = {
  login() {
    return 'logged in'
  },
}

const admin = Object.create(user)
admin.role = 'admin'

console.log(admin.login()) // logged in

admin does not directly have login.

JavaScript looks up the prototype chain and finds login on user.

Prototype chain lookup

When you access:

admin.login

JavaScript checks:

1. Does admin have login?
2. If not, check admin's prototype.
3. Continue until found or chain ends.

Classes use prototypes

JavaScript class syntax is mostly cleaner syntax over prototypes.

class User {
  login() {
    return 'logged in'
  }
}

Methods like login live on User.prototype.

Interview answer

JavaScript uses prototype-based inheritance. When a property is not found on an object, JavaScript looks up its prototype chain. Class syntax in JavaScript is built on top of this prototype system.