instanceof的原理及实现

instanceof的用途是判断一个对象是否在某个对象原型链上。或者说判断一个对象是某个对象的实例
这里的对象指的是Object或Array或Function

let a = function(){
    
    }
function myInstanceof(left, right) {
    
    
    if (left === undefined || left === null) return false
    let leftProto = left.__proto__,
        rightProto = right.prototype
    while (true) {
    
    
        if (leftProto === null) {
    
    
            return false
        }
        if (leftProto === rightProto) {
    
    
            return true
        }
        leftProto = leftProto.__proto__
    }
}
console.log(myInstanceof(a, Function));