SOURCE

// call方法实现
Function.prototype.Mycall = function(context) {
    if(typeof this != "function") {
        throw new Error("type error")
    }
    context = context || window;
    let arr = [...arguments].slice(1);
    let result = null;

    let fn = Symbol();
    context[fn] = this;
    result = context[fn](...arguments);
    delete context.fn;
    return result;
}

var obj = {
  value: "vortesnail",
};

function fn() {
  console.log(this.value);
}

fn.Mycall(obj); // vortesnail

// apply方法的实现
Function.prototype.Myapply = function(context) {
    if(typeof this != "function") {
        throw new Error("type error");
    }
    context = context || window;
    let result = null;
    let fn = Symbol();
    context[fn] = this;
    if(arguments[1]) {
        result = context[fn](arguments[1]);
    }else {
        result = context[fn]();
    }
    delete context.fn;
    return result;
}

let a = {};
let c = Symbol();
a[c] = function() {
    console.log(1);
}
a[c]();
console.log(a);

// bind实现
Function.prototype.Mybind = function(context) {
    if(typeof this != "function") {
        throw new Error("type error");
    }
    let arr = [...arguments].slice(1);
    let fn = this;
    return function Fn() {
        return fn.apply(
            this instanceof Fn ? this : context,
            arr.concat(...arguments)
        )
    }
}