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O operador instanceof testa se um objeto tem, em seu prototype, a função construtora.


objeto instanceof construtor


O objeto a ser testado
Função construtora a ser verificada


O operador instanceof testa a presença da função construtora no prototype do objeto.

// defenindo construtores
function C(){}
function D(){}

var o = new C();

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

// false, porque D.prototype não está no prototype desse objeto
o instanceof D;

o instanceof Object; // true, porque:
C.prototype instanceof Object // true

C.prototype = {};
var o2 = new C();

o2 instanceof C; // true

// false, porque C.prototype não está mais no prototype desse objeto
o instanceof C; 

D.prototype = new C(); // use inheritance
var o3 = new D();
o3 instanceof D; // true
o3 instanceof C; // true

Note que o resultado do instanceof pode alterar quando a gente altera o prototype da função construtora. No entanto, a gente não pode alterar (por padrão) o prototype do objeto. Só é possível fazer essa alteração usando a pseudopropriedade __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;    // retorna true
var b = mycar instanceof Object; // retorna true


Especificação Status Comentário
ECMAScript 1st Edition. Standard Definição inicial. Implementada no JavaScript 1.4
ECMAScript 5.1 (ECMA-262)
The definition of 'The instanceof operator' in that specification.
ECMAScript 6 (ECMA-262)
The definition of 'Relational Operators' in that specification.
Release Candidate  

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