Overview

• In JavaScript, objects have a special hidden property [[Prototype]], that is either null or references another object.
  • That object is called “a prototype”:
• That [[Prototype]] has a “magical” meaning.
• When we want to read a property from object, and it’s missing, JavaScript automatically takes it from the prototype.
• In programming, such thing is called “prototypal inheritance”.
• The property [[Prototype]] is internal and hidden, but there are many ways to set it.
• Note that __proto__ is not the same as [[Prototype]].
  • __proto__ is a historical getter/setter for [[Prototype]]
• It exists for historical reasons, in modern language it is replaced with functions Object.getPrototypeOf/Object.setPrototypeOf that also get/set the prototype.

let animal = {
  eats: true
};
let rabbit = {
  jumps: true
};
// Sets animal to be a prototype of rabbit.
rabbit.__proto__ = animal;

// we can find both properties in rabbit now:
console.log(rabbit.eats); // "true" from animal
console.log(rabbit.jumps); // true

• If we have a method in animal, it can be called on rabbit:

let animal = {
  eats: true,
  walk() {
    alert("Animal walk");
  }
};

let rabbit = {
  jumps: true,
  __proto__: animal
};

// walk is taken from the prototype
rabbit.walk(); // Animal walk

• There are actually only two limitations:

1. The references can’t go in circles.
   • JavaScript will throw an error if we try to assign __proto__ in a circle.
2. The value of __proto__ can be either an object or null, other types (like primitives) are ignored.
   • Also it may be obvious, but still: there can be only one [[Prototype]].
   • An object may not inherit from two others.

• The prototype is only used for reading properties.
• Write/delete operations work directly with the object.
• That’s for data properties only, not for accessors.
  • If a property is a getter/setter, then it behaves like a function: getters/setters are looked up in the prototype.

The value of “this”

• An interesting question may arise in the example above: what’s the value of this inside set fullName(value)?
• Where the properties this.name and this.surname are written: into user or admin?