简单的实现instanceOf

	Function.prototype.ownInstanceOf = function(instanceObj, prototypeObj) {
    
    
		let current = instanceObj.__proto__
		while (current) {
    
    
			if (current === prototypeObj.prototype)return true
			current = current.__proto__
		}
		return false
	}
	function Foo() {
    
    }

	let foo = new Foo()
	Function.ownInstanceOf(foo, Foo)

这里instanceof主要就是找第一个参数的原型链上是否存在第二个参数的原型对象