JS 手写 instanceof

╰+攻爆jí腚メ 2022-10-31 12:24 189阅读 0赞

### JS 手写 instanceof ###

<!DOCTYPE html>
    <html lang="en">
    
    <head>
        <meta charset="UTF-8">
        <meta http-equiv="X-UA-Compatible" content="IE=edge">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Document</title>
    </head>
    
    <body>
        <script>
            //自定义封装instanceof(A,B) 判断B的原型对象在不在A的原型链上 
            // 类似于 A instanceof B
            function myInstanceof(A, B) { 
                //首先先获取B的原型对象
                var BP = B.prototype;
                //只要A沿着原型链 能找到B 那就返回true 否则返回false
                //while判断 每次循环的时候A会发生变化
                while (A) { 
                    //判断当前的A的原型链上有没有BP
                    if (A.__proto__ === BP) { 
                        return true;
                    }
                    //把A变化为A的上一级原型链
                    A = A.__proto__;
                }
    
                //只要while没有执行return true 并且退出while了
                // 说明B的原型对象不在A的原型链上
                return false;
            }
    
       
            console.log(myInstanceof(Object, Function));
            console.log(myInstanceof(Object, Object));
            console.log(myInstanceof(Function, Object));
            console.log(myInstanceof(Function, Function));
            console.log(myInstanceof(Object, Array));
        </script>
    </body>
    
    </html>