new的实现

// 原生js实现

function _new() { 
        const { constructor, arg } = arguments[0]; 
        if (typeof constructor !== 'function') {
            throw 'first param must be function'
        }
        let target = {}; 
        target.__proto__ = constructor.prototype;
        let result = constructor.apply(target, [arg]); 
        if (result &&  (typeof result === 'object' || typeof result === 'function')) {
            // 如果构造函数返回一个对象，那就返回这个对象
            return result
        }
        return target 
   }

 

//  借助es6实现

function _new(){
        const { constructor, arg } = arguments[0]; 
        if(typeof constructor !== 'function'){
            throw 'newOperator function the first param must be a function';
        }
        // es6中为target指向constructor
        _new.target = constructor;

        // es6实现
        var newObj = Object.create(constructor.prototype);
        var result = constructor.apply(newObj, [arg]);

        if (result &&  (typeof result === 'object' || typeof result === 'function')) {
            // 如果构造函数返回一个对象，那就返回这个对象
            return result
        }
        return newObj;
    }

// 验证

function A(b) {this.b = b}
A.prototype.c = function(){console.log('this.b',this.b)}

let a = _new({
     constructor: A,
     arg: 'b'
})
a.c()