JavaScript/Notes/Prototype

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The prototype chain is used by JavaScript engines for reading property resolution.  
 
The prototype chain is used by JavaScript engines for reading property resolution.  
  
User-defined [[JavaScript/Notes/Functions functions]] can be designed for use as constructors, to create new objects. All objects have, on their prototype chain, their constructor's prototype.  
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User-defined [[JavaScript/Notes/Function#Functions_Double_as_Constructor|functions]] can be designed for use as constructors to create new objects which then have, on their prototype chain, that constructor function's `prototype`. Review from [[JavaScript/Notes/Function#Functions_Double_as_Constructor|Functions]]:
 +
{{Function Constructor}}
  
 
Define constructor functions how assigning `prototype` property to the constructor creates the prototype chain, for shared properties on the constructor function's object instances.  
 
Define constructor functions how assigning `prototype` property to the constructor creates the prototype chain, for shared properties on the constructor function's object instances.  

Latest revision as of 12:13, 12 August 2014

The prototype chain is used by JavaScript engines for reading property resolution.

User-defined functions can be designed for use as constructors to create new objects which then have, on their prototype chain, that constructor function's `prototype`. Review from Functions:

// Colorized is a constructor.
function Colorized(name, favoriteColor) {
  this.name = name;
  this.favoriteColor = favoriteColor;
}
 
// c is an instance of Colorized.
var c = new Colorized("Mary", "red");
alert(c.toString());

A toString method is available on every native object. This is because toString is defined on Object.prototype and Object.prototype is at the base the prototype chain.

Define constructor functions how assigning `prototype` property to the constructor creates the prototype chain, for shared properties on the constructor function's object instances.

WebIDL defines the prototype chain to describe native inheritance of host object interfaces.

Contents

[edit] Prototype Chain

Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of its constructor’s prototype property.


[edit] Constructors

Functions are used for constructors, to instantiate new objects.

function MyConstructor() {
 
}
 
alert(MyConstructor.prototype); // Look what we get for free!

When any function is used in a new Expression, a new object is created and given a link to the constructor function's `prototype` property (The new Operator § 11.2.2).

function MyConstructor() {
 
}
 
var anObj = new MyConstructor; // [[Prototype]] comes from MyConstructor.prototype

Shared Properties

Every user-defined function is given a prototype property by the implementation. This property has a constructor property that points back to the constructor function.

function MyConstructor() {
 
}
 
alert(MyConstructor.prototype.constructor);

The constructor's prototype property can be modified, and this allows for shared properties for each new instance created by that constructor.

function MyConstructor(id) {
  this.id = id;
}
 
MyConstructor.prototype.method = function() {
  return this.id;
};
 
alert(new MyConstructor("h").method());


Internal Prototype Properties

All objects have an internal property called [[Prototype]]. The value of this property is either null or an object and is used for implementing inheritance. Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) for the purposes of get access, but not for put access. (§ 8.6.2)

function MyConstructor() {
 
}
 
MyConstructor.prototype = {
    toString : function() {
      return "[object MyConstructor]";
    }
};
 
alert(new MyConstructor().toString())

As shown in the above example, any function's prototype property can be replaced by any user-defined object.

[edit] Subclassing

Why Subclass?

function MyConstructor(name) {
}
 
MyConstructor.prototype = {
    toString : function() {
      return "[object MyConstructor]";
    }
};
 
function MySubclass(name) { }
MySubclass.prototype = new MyConstructor;
// Base class.
function MyConstructor(name) {
  this.name = name;
}
 
// Prototype.
MyConstructor.prototype = {
    toString : function() {
      return "[object MyConstructor]";
    }
};
 
// Subclass.
function MySubclass(name) { 
// Call super constructor.
  MyConstructor.call(this, name);
}

JSBin Example: http://jsbin.com/upuQAtAV/1/edit

[edit] Shadowing

(whiteboard diagram)

[edit] Object.create

// Extend prototype.
MySubclass.prototype = Object.create(MyConstructor.prototype);
 
MySubclass.prototype.valueOf = function() {
  return this.name;
};

See also: Object.create() | MDN A more complex Prototype Chain inheritance explanation, example, and diagram.

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