JavaScript operator instanceof

Overview

instanceof Operator can be used to determine the function of a constructorprototype属性是否存在另外一个要检测对象的原型链上。

Grammar

instanceof Constructor

Parameters

object

the object to be detected.

constructor

a constructor

Describe

instanceofoperator is used to detect the constructor.prototype existence object of a prototype chain on a parameter.

functionC () {}//define a constructorfunctionD () {}//define another constructorvarn \NewC (); OinstanceofC//true because: object.getprototypeof (o) = = = C.prototypeOinstanceofD//false, because D.prototype is not on the prototype chain of OOinstanceofObject;//true, because Object.prototype.isPrototypeOf (o) returns TrueC.prototypeinstanceofObject//true, Ibid.C.prototype= {};varO2 =NewC (); O2instanceofC//trueOinstanceofC//False,c.prototype points to an empty object that is not on the prototype chain of O.D.prototype=NewC ();varO3 =NewD (); O3instanceofD//trueO3instanceofC//true

It should be noted that if the expression obj instanceof Foo 返回true，则并不意味着该表达式会永远返回ture，因为Foo.prototype属性的值有可能会改变 , the value after the change is likely to not exist in obj the prototype chain, then the value of the original expression will become false . In another case, the value of the original expression will change, that is, changing the obj prototype chain of the object, although in the current ES specification, we can only read the prototype of the object can not change it, but with the help of non-standard __proto__魔法属性 , 是可以实现的 . For example 执行obj.__proto__ = {}之后 ,obj instanceof Foo就会返回false了。

instanceof和多全局对象(Interaction between multiple frame or multiple windows)

In the browser, our script may need to interact between multiple windows. Multiple windows mean multiple global environments, and different global environments have different global objects, with different built-in type constructors. This may cause some problems. For example, the expression [] instanceof window.frames[0].Array returns false , because Array.prototype !==  window.frames[0].Array .prototype , 因此你必须使用 Array.isArray(myObj)或者 Object.prototype.toString.call(myObj) === "[object Array]" to determine whether MyObj is an array.

Example Example: Indicates StringObjects, and Date对象都属于 Object类型

The following code is used instanceof to prove: String和 Date对象 also belongs to Object类型 .

varMyString =NewString ();varMyDate =NewDate (); MyStringinstanceofString;//returns TrueMyStringinstanceofObject;//returns TrueMyStringinstanceofDate;//returns falsemydateinstanceofDate;//returns TrueMyDateinstanceofObject;//returns TrueMyDateinstanceofString;//returns false

Example: Indicates mycar属于 Car类型,同时又Belong to Object类型

The following code creates a type Car 以及该类型的对象实例 mycar . instanceof operator indicates this mycar对象既属于 Car类型 , 又属于 Object类型 .

function Car (make, model, year) {  the. Make = make ;    this. Model = model;   this. Year = year ;} var New Car ("Honda", "Accord", 1998); var instanceof Car;    // returns True var instanceof // returns True

Specification

Specification	Status	Comment
ECMAScript 1st Edition.	Standard	Initial definition. Implemented in JavaScript 1.4
ECMAScript 5.1 (ECMA-262) The instanceof operator	Standard
ECMAScript 6 (ECMA-262) Relational Operators	Draft

Browser compatibility

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic Support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)

JavaScript operator instanceof