instanceof - JavaScript 编辑

The instanceof operator tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value. 

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Syntax

object instanceof constructor

Parameters

object

The object to test.

constructor

Function to test against

Description

The instanceof operator tests the presence of constructor.prototype in object's prototype chain.

// defining constructors
function C() {}
function D() {}

let o = new C()

// true, because: Object.getPrototypeOf(o) === C.prototype
o instanceof C

// false, because D.prototype is nowhere in o's prototype chain
o instanceof D

o instanceof Object           // true, because:
C.prototype instanceof Object // true

C.prototype = {}
let o2 = new C()

o2 instanceof C  // true

// false, because C.prototype is nowhere in
// o's prototype chain anymore
o instanceof C

D.prototype = new C()  // add C to [[Prototype]] linkage of D
let o3 = new D()
o3 instanceof D        // true
o3 instanceof C        // true since C.prototype is now in o3's prototype chain

Note that the value of an instanceof test can change based on changes to the prototype property of constructors. It can also be changed by changing an object's prototype using Object.setPrototypeOf. It is also possible using the non-standard __proto__ property.

instanceof and multiple context (e.g. frames or windows)

Different scopes have different execution environments. This means that they have different built-ins (different global object, different constructors, etc.). This may result in unexpected results. For instance, [] instanceof window.frames[0].Array will return false, because Array.prototype !==window.frames[0].Array and arrays inherit from the former.

This may not make sense at first, but for scripts dealing with multiple frames or windows, and passing objects from one context to another via functions, this will be a valid and strong issue. For instance, you can securely check if a given object is, in fact, an Array using Array.isArray(myObj)

For example, checking if a Nodes is a SVGElement in a different context, you can use myNode instanceof myNode.ownerDocument.defaultView.SVGElement.

Note for Mozilla developers:

In code using XPCOM, instanceof has special effect: obj instanceof xpcomInterface (e.g. Components.interfaces.nsIFile) calls obj.QueryInterface(xpcomInterface) and returns true if QueryInterface succeeded.

A side effect of such call is that you can use xpcomInterface's properties on obj after a successful instanceof test. Unlike standard JavaScript globals, the test obj instanceof xpcomInterface works as expected, even if obj is from a different scope.

Examples

Demonstrating that String and Date are of type Object and exceptional cases

The following code uses instanceof to demonstrate that String and Date objects are also of type Object (they are derived from Object).

However, objects created with the object literal notation are an exception here: Although the prototype is undefined, instanceof Object returns true.

let simpleStr = 'This is a simple string'
let myString  = new String()
let newStr    = new String('String created with constructor')
let myDate    = new Date()
let myObj     = {}
let myNonObj  = Object.create(null)

simpleStr instanceof String  // returns false, string literal is not an object
myString  instanceof String  // returns true
newStr    instanceof String  // returns true
myString  instanceof Object  // returns true

myObj    instanceof Object   // returns true, every object literal has Object.prototype as prototype
({})     instanceof Object   // returns true, same case as above
myNonObj instanceof Object   // returns false, prototype is end of prototype chain (null)

myString instanceof Date     // returns false

myDate instanceof Date      // returns true
myDate instanceof Object    // returns true
myDate instanceof String    // returns false

Demonstrating that mycar is of type Car and type Object

The following code creates an object type Car and an instance of that object type, mycar. The instanceof operator demonstrates that the mycar object is of type Car and of type Object.

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}
let mycar = new Car('Honda', 'Accord', 1998)
let a = mycar instanceof Car     // returns true
let b = mycar instanceof Object  // returns true

Not an instanceof

To test if an object is not an instanceof a specific constructor, you can do

if (!(mycar instanceof Car)) {
  // Do something, like:
  // mycar = new Car(mycar)
}

This is really different from:

if (!mycar instanceof Car)

This will always be false. (!mycar will be evaluated before instanceof, so you always try to know if a boolean is an instance of Car).

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• typeof
• Symbol.hasInstance
• Object.prototype.isPrototypeOf