instanceof的实现

这张图对原型、原型链、继承的理解非常重要

instanceof的判定规则

function Person(name) {
	this.name = name;
}
const p = new Person("tianzhen");
p instanceof Person; // true
p instanceof Object; // true

等价于

• p.__proto__ === Person.prototype
• p.__proto__.__proto__ === Object.prototype

  如果查不到就到原型的原型查找，一直到原型链的顶端

实现1

function _instanceof(Left, Right) {
	if (typeof Left !== "object" || Left === null) {
		return false;
	}

	while (true) {
		if (Left === null) {
			return false;
		}
		if (Left.__proto__ === Right.prototype) {
			return true;
		}
		Left = Left.__proto__;
	}
}

__proto__并不是标准

实现2

function _instanceof(Left, Right) {
	if (typeof Left !== "object" || Left === null) {
		return false;
	}

	let L = Object.getPrototypeOf(Left);
	while (true) {
		if (L === null) {
			return false;
		}
		if (L === Right.prototype) {
			return true;
		}
		L = Object.getPrototypeOf(L);
	}
}