javascript - JavaScript中instanceof是如何实现的

Question

让我们考虑以下代码片段:

function A() {}
var obj = new A();
function B() {};
obj.constructor = B;
console.info("1: ", obj.constructor); //Function B
console.info("2: ", obj instanceof A); //true
console.info("3: ", obj instanceof B); //false

我的猜测是，要确定对象是否是某个函数类的实例，JS 引擎必须检查该对象是否具有相同的构造函数属性。但它似乎并没有发生，因为即使覆盖对象的构造函数属性也不会改变它的 instanceof 输出。

Answer 1

ECMAScript 262 规范会告诉您这一点。直接引用其The `instanceof operator部分:

11.8.6 The instanceof operator

The production RelationalExpression : RelationalExpression instanceof ShiftExpression is evaluated as follows:

1. Let lref be the result of evaluating RelationalExpression.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating ShiftExpression.
4. Let rval be GetValue(rref).
5. If Type(rval) is not Object, throw a TypeError exception.
6. If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
7. Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

至于[[HasInstance]]是什么，规范也在其Object Internal Properties and Methods中进行了定义。部分为:

[[HasInstance]]

Returns a Boolean value indicating whether the argument is likely an Object that was constructed by this object. Of the standard built-in ECMAScript objects, only Function objects implement [[HasInstance]]

进一步阅读:http://www.ecma-international.org/ecma-262/5.1/#sec-15.3.4.5.3