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instanceof

2016-04-22 12:59 148 查看
The 
instanceof
operator tests whether an object has in its prototype chain the 
prototype
property of a constructor.

Syntax

object instanceof constructor

Parameters

object
The object to test.
constructor
Function to test against

Description

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

// 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


Note that if the value of an 
instanceof
test can change based on changes to the 
prototype
property of constructors, it cannot be changed by changing an object prototype, because changing an object prototype is not possible in standard ECMAScript. It is however possible using the non-standard 
__proto__
pseudo-property.

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 instanceofxpcomInterface
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;    // returns true
var b = mycar instanceof Object; // returns true


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