JavaScript Prototype Pattern


Example

The prototype pattern focuses on creating an object that can be used as a blueprint for other objects through prototypal inheritance. This pattern is inherently easy to work with in JavaScript because of the native support for prototypal inheritance in JS which means we don't need to spend time or effort imitating this topology.


Creating methods on the prototype

function Welcome(name) {
  this.name = name;
}
Welcome.prototype.sayHello = function() {
  return 'Hello, ' + this.name + '!';
}

var welcome = new Welcome('John');

welcome.sayHello();
// => Hello, John!

Prototypal Inheritance

Inheriting from a 'parent object' is relatively easy via the following pattern

ChildObject.prototype = Object.create(ParentObject.prototype);
ChildObject.prototype.constructor = ChildObject;

Where ParentObject is the object you wish to inherit the prototyped functions from, and ChildObject is the new Object you wish to put them on.

If the parent object has values it initializes in it's constructor you need to call the parents constructor when initializing the child.

You do that using the following pattern in the ChildObject constructor.

function ChildObject(value) {
    ParentObject.call(this, value);
}

A complete example where the above is implemented

function RoomService(name, order) {
  // this.name will be set and made available on the scope of this function
  Welcome.call(this, name);
  this.order = order;
}

// Inherit 'sayHello()' methods from 'Welcome' prototype
RoomService.prototype = Object.create(Welcome.prototype);

// By default prototype object has 'constructor' property. 
// But as we created new object without this property  -  we have to set it manually,
// otherwise 'constructor' property will point to 'Welcome' class
RoomService.prototype.constructor = RoomService;

RoomService.prototype.announceDelivery = function() {
  return 'Your ' + this.order + ' has arrived!';
}
RoomService.prototype.deliverOrder = function() {
  return this.sayHello() + ' ' + this.announceDelivery();
}

var delivery = new RoomService('John', 'pizza');

delivery.sayHello();
// => Hello, John!,

delivery.announceDelivery();
// Your pizza has arrived!

delivery.deliverOrder();
// => Hello, John! Your pizza has arrived!