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:

1. 如果V不是对象，则返回false。


2. ....

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.