call/apply/bind

• call

  Function.prototype.myCall = function (obj, ...arg) {
    const context = obj || window;
    const fn = Symbol(); // 保证唯一
    context[fn] = this;
    const res = context[fn](...arg);
    delete context[fn];
    return res;
  }

• apply

  Function.prototype.myApply = function (obj, arg) {
    const context = obj || window;
    const fn = Symbol(); // 保证唯一
    context[fn] = this; 
    const res = context[fn](...arg);
    delete context[fn];
    return res;
  }

• bind ```javascript Function.prototype.bind = function (obj, …args) { const context = obj || window; const fn = Symbol() context[fn] = this; let that = this;

  const res = function(…others){

      return that.apply(
      this instanceof that ? this : context, //new时， this指向res的实例，res继承自that
    [...args,...others]
  )

  }

  // 如果绑定的是构造函数 那么需要继承构造函数原型属性和方法 res.prototype = Object.create(that.prototype)

  return res }

// 使用 function Point(x, y) { this.x = x; this.y = y; }

Point.prototype.toString = function () { return this.x + ‘,’ + this.y; }

let YPoint = Point.bind(null, 1); let axiosPoint = new YPoint(2);

console.log(axiosPoint.toString()) // ‘1,2’ ```