手写call、apply、bind函数

            var year = 2021
            function getDate(month, day) {
                return this.year + '-' + month + '-' + day
            }

            Function.prototype.myCall = function (context, ...args) {
                // constext 如果没传入就设置为window
                context = context || window;
                // 将调用函数设置为对象方法
                context.fn = this;
                // 调用函数
                const result = context.fn(...args);
                // 将属性删除
                delete context.fn;
                return result;
            }
            Function.prototype.myApply = function (context, args) {
                context = context || window;
                context.fn = this;
                const result = args ? context.fn(...args) : context.fn();
                delete context.fn;
                return result;
            }
            Function.prototype.myBind = function (context, ...args) {
                context = context || window;
                const that = this;
                return function (...args) {
                    context.fn = that;
                    const result = context.fn(...args);
                    delete context.fn;
                    return result
                }
            }

            var obj = { year: 2000 }
            console.log('myCall', getDate.myCall(null, 3, 8))
            console.log('myApply', getDate.myApply(obj, [4, 8])) 
            console.log('myBind', getDate.myBind(obj)(3, 8))