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Оператор instanceof  проверяет к какому классу принадлежит объект.


object instanceof constructor


Проверяемый объект.
Искомая функция.


Оператор instanceof проверяет присутствие конструктора в цепочке прототипов объекта.

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

var 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 = {};
var 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(); // use inheritance
var o3 = new D();
o3 instanceof D; // true
o3 instanceof C; // true

Обратите внимание: если значение проверки instanceof может меняться в зависимости от изменений прототипной принадлежности конструкторов, его нельзя изменить изменением прототипа объекта, поскольку изменение прототипа объекта невозможно в стандартном ECMAScript. Однако, это можно сделать, используя нестандартное псевдо-свойство __proto__ .

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

Different scope 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 when you start dealing with multiple frames or windows in your script and pass 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)

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.


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.

var simpleStr = "This is a simple string"; 
var myString  = new String();
var newStr    = new String("String created with constructor");
var myDate    = new Date();
var myObj     = {};

simpleStr instanceof String; // returns false, checks the prototype chain, finds undefined
myString  instanceof String; // returns true
newStr    instanceof String; // returns true
myString  instanceof Object; // returns true

myObj instanceof Object;    // returns true, despite an undefined prototype
({})  instanceof Object;    // returns true, same case as above

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;
var mycar = new Car("Honda", "Accord", 1998);
var a = mycar instanceof Car;    // returns true
var b = mycar instanceof Object; // returns true


Specification Status Comment
ECMAScript 2017 Draft (ECMA-262)
Определение 'Relational Operators' в этой спецификации.
ECMAScript 2015 (6th Edition, ECMA-262)
Определение 'Relational Operators' в этой спецификации.
ECMAScript 5.1 (ECMA-262)
Определение 'The instanceof operator' в этой спецификации.
ECMAScript 3rd Edition (ECMA-262)
Определение 'The instanceof operator' в этой спецификации.
Стандарт Initial definition. Implemented in JavaScript 1.4.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Да) (Да) (Да) (Да) (Да)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Да) (Да) (Да) (Да) (Да) (Да)

See also

Метки документа и участники

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