JavaScript/Notes/Prototype: Difference between revisions
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== Prototype Chain == | == Prototype Chain == | ||
<blockquote>Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of its constructor’s “prototype” property | <blockquote>Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of its constructor’s “prototype” property. | ||
</blockquote> | </blockquote> | ||
Revision as of 20:43, 7 January 2014
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.
Constructors
<source lang="javascript"> function MyConstructor() {
}
alert(MyConstructor.prototype); // Look what we get for free! </source>
<source lang="javascript"> function MyConstructor() {
}
var anObj = new MyConstructor; // Prototype comes from MyConstructor.prototype </source>
Every user-defined function has a prototype property, provided by the implementation. The prototype property that is provided by the implementation has a constructor
property that points back to the constructor function.
<source lang="javascript"> function MyConstructor() {
}
alert(MyConstructor.prototype.constructor); </source>
<source lang="javascript"> function MyConstructor(id) {
this.id = id;
}
MyConstructor.prototype.method = function() {
return this.id;
};
alert(new MyConstructor("h").method()); </source>
The prototype property is used for prototype inheritance of shared properties. 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.
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)
<source lang="javascript"> function MyConstructor() {
}
MyConstructor.prototype = {
toString : function() { return "[object MyConstructor]"; }
};
alert(new MyConstructor().toString()) </source> As shown in the above example, any function's prototype property can be replaced by any user-defined object.
Subclassing
Why Subclass? <source lang="javascript"> function MyConstructor(name) { }
MyConstructor.prototype = {
toString : function() { return "[object MyConstructor]"; }
};
function MySubclass(name) { } MySubclass.prototype = new MyConstructor; </source>
<source lang="javascript"> // 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);
} </source>
JSBin Example: http://jsbin.com/upuQAtAV/1/edit
Shadowing
(whiteboard diagram)
Object.create
<source lang="javascript">// Extend prototype. MySubclass.prototype = Object.create(MyConstructor.prototype);
MySubclass.prototype.valueOf = function() {
return this.name;
}; </source>
See also: Object.create() | MDN A more complex Prototype Chain inheritance explanation, example, and diagram.