instanceof
instanceof
testa se um objeto tem, em seu prototype, a função construtora.Experimente
Sintaxe
objeto instanceof construtor
Parâmetros
objeto
-
O objeto a ser testado
construtor
-
Função construtora a ser verificada
Descrição
O operador instanceof testa a presença da função construtora no prototype do objeto.
// definindo 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)
Nota: 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
.
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
Especificações
Specification |
---|
ECMAScript Language Specification # sec-relational-operators |
Compatibilidade com navegadores
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