使用instanceof和检查构造函数之间有什么区别? [英] What's the difference between using instanceof and checking the constructor?
问题描述
为什么以下两行返回不同的结果?
Why do the following two lines return different results?
("test" instanceof String) // returns false
("test".constructor == String) // returns true
在控制台中测试chrome版本28.0.1500.95 m
Tested in the console of chrome version 28.0.1500.95 m
本机类型的工作方式是否略有不同?
Does it work slightly differently for native types?
推荐答案
构造函数
只是内部 [[prototype]]
属性的属性,可以轻松实现操纵:
constructor
is just a property of the internal [[prototype]]
property, that can easily be manipulated:
function A(){}
function B(){}
A.prototype.constructor = B;
var a = new A();
console.log(a.constructor); //B
instanceof
运算符然而检查即使您更改了构造函数的完整 prototype
属性,内部原型链并不容易被愚弄:
The instanceof
operator however checks the internal prototype chain and is not so easily to be fooled, even if you change the complete prototype
property of the constructor function:
function A(){}
function B(){}
A.prototype = B.prototype;
var a = new A();
console.log(a instanceof A); //true
console.log(a instanceof B); //false
那么,为什么testinstanceof String === false
但是(test.constructor == String)=== true
?
首先,test
是一个原语,原语永远不是任何实例。使用 instanceof
时实际发生的是构造函数的内部 [[HasInstance]]
方法被调用可能的实例作为参数。所以 A
的实例大致翻译为:
First of all, "test"
is a primitive and primitives are never an instance of anything. What actually happens when you use instanceof
is that the internal [[HasInstance]]
method of the constructor is called with the possible instance as an argument. So a instanceof A
translates roughly to:
`A.[[HasInstance]](a)`
ECMA规范要求 [[HasInstance]]
: http ://www.ecma-international.org/ecma-262/5.1/#sec-15.3.5.3
The ECMA Specs have this to say to [[HasInstance]]
: http://www.ecma-international.org/ecma-262/5.1/#sec-15.3.5.3
[[HasInstance]](V)
假设F是一个Function对象。
Assume F is a Function object.
当使用值V,
调用F的[[HasInstance]]内部方法时,将执行以下步骤:
When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:
- 如果V不是对象,则返回false。
- ....
换句话说:如果 instanceof
的左侧不是对象,运算符将返回false。
In other words: If the left hand side of instanceof
is not an object, the operator will return false.
(test.constructor == String)=== true
works f或者不同的原因:如果您尝试访问基元的属性,则基元将临时转换为对象。所以test.constructor
大致等于:
("test".constructor == String) === true
works for a different reason: If you try to access a property of a primitive, the primitive will be temporarily converted into an object. So "test".constructor
is roughly equal to:
(new String("test")).constructor
在这种情况下,您实际上是在创建一个对象,构造函数,然后请求构造函数
属性。因此毫不奇怪,它将返回 String
。
in which case you are actually creating an object, with a constructor function and requesting the constructor
property afterward. So it is no surprise, that it will return String
.
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