Prototype (Prototypal inheritance)

A prototype is an object from which other objects can inherit properties and methods.
Every object in JavaScript has an internal link [[Prototype]], which points to another object or null.

Prototype Chain

• If a property is not found in an object, the search continues through its prototype chain.
• This chain forms the inheritance mechanism known as the prototype chain.
• The end of the chain is Object.prototype, after which comes null.

const obj = {};
console.log(obj.__proto__ === Object.prototype); // true

Constructor Functions and prototype

• Every function in JS has a prototype property.
• Objects created via new inherit properties and methods from Function.prototype.

function User(name) {
  this.name = name;
}
User.prototype.sayHi = function() {
  return `Hi, ${this.name}`;
};

const user = new User('Alex');
console.log(user.sayHi()); // 'Hi, Alex'

Object.create()

Creates a new object with the specified prototype:

const person = { greet() { return 'Hello'; } };
const user = Object.create(person);
console.log(user.greet()); // 'Hello'

Key Idea

• A prototype is a mechanism for delegating properties.
• All objects inherit from Object.prototype unless specified otherwise.
• The prototype model is at the core of classes (class) and inheritance in modern JavaScript.

Links

JavaScript Object Prototypes — Medium