Confused by javascript's constructor and prototype?

function 开发者_JAVA百科MyObject(){}
Array.prototype={};
MyObject.prototype={};
var a=new Array();
var b=new MyObject();
alert(a.constructor==Array);//true
alert(b.constructor==MyObject);//false

Array.prototype is a non-writable property.

As such, your assignment:

Array.prototype = {}

...doesn't succeed, and so its .constructor property hasn't changed.

15.4.3.1 Array.prototype

The initial value of Array.prototype is the Array prototype object (15.4.4).

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }

...whereas with your custom constructor, you have the ability to assign a different prototype object, so you've overwritten the original which had reference to the constructor via .constructor.

The constructor property is overwritten when you override the prototype property with your own empty object instance, since ({}).constructor === Object. You can do either

function MyObject() {}
MyObject.prototype = {};
MyObject.prototype.constructor = MyObject;

or (better IMO) you can not set prototype directly, but instead augment it:

function MyObject() {}
MyObject.prototype.foo = "bar";

Also of note: Array.prototype is not writable, so your line Array.prototype = {} will silently fail (or noisily fail in strict mode).

> function MyObject(){} 
> Array.prototype={};

You can't assign a value to Array.prototype.

> MyObject.prototype={};
> var a=new Array();
> var b=new MyObject();
> alert(a.constructor==Array);//true

Array.prototype has a constructor property that references the Array function. Since a is an instance of Array, it inherits Array.prototype's constructor property.

> alert(b.constructor==MyObject);//false

You have assigned an empty object to MyObject.prototype, it does not have a prototype property, nor does b.

MyObject.prototype.constructor = MyObject;

alert(b.constructor==MyObject); // true