各种 polyfill 问题 new call 等

[dave-wind]

模拟实现 new 关键字功能

function fakerNew() {
 var args = arguments;
// 截取出 构造函数; 剩下的args 为剩余参数
 var constructor = Array.prototype.shift.apply(arguments);
// 模拟实现 原型指针 指向; 定义一个 obj
var obj = {};
obj.__proto__ = constructor.prototype;
//  把obj作为 参数传入 改变this 指向;
 var res = constructor.apply(obj,args);

return res instanceof Object? res: obj
}

function Person(name) {
    this.name = name;
}
var demo = fakerNew(Person,'dave');
console.log(demo); //  Person {name:'dave'}  实例

[dave-wind]

Bind 函数

Function.prototype._bind = function(ctx,...args) {
   const self = this; // 谁调用 就是指向谁
    return function(){
        //  self.call(self,...args)
      // 如果有返回值 需要return出去
       return self.call(self,...args)
    }
}
eg:
const obj = {
    name: 'dave',
    show() {
        console.log(this.name)
    }
}
const obj2 = { name: 'wind' };
var a= obj.show.bind(obj2);
a(); // wind

[dave-wind]

instanceof polyfill

function A() { }
var a1 = new A();

function _instanceof(target, Fn) {
    if ((typeof target !== 'object' && typeof target !== 'function') || target == undefined) {
        return false
    }
    let proto = target.__proto__;
    //  层层往上找
    while (true) {

        if (proto === null) return false

        if (proto === Fn.prototype) {
            return true
        }
        // 如果不是，继续向上找
        proto = proto.__proto__;
    }

}

console.log("A----", _instanceof(a1, A));

[dave-wind]

map

Array.prototype._map = function (exec) {
    const result = [];
    for (let i = 0; i < this.length; i++) {
        result[i] = exec(this[i], i, this)
    }
    return result;
}

const a2 = new Array(5).fill(1)
console.log(a2, a2._map((itm, index) => itm * index + 1)) // [1,2,3,4,5]

[dave-wind]

Obejct.create

Object.prototype._create = function (proto) {
  const Fn = function () { }
  Fn.prototype = proto
  return new Fn()
}
function A() { }
const obj = Object.create(A)
const obj2 = Object._create(A)
console.log(obj.__proto__ === A) // true
console.log(obj.__proto__ === A) // true

[dave-wind]

Filter

Array.prototype._filter = function (fn) {
    let result = [];
    for (let i = 0; i < this.length; i++) {
      // 过滤执行结果
        if (fn(this[i], i, this)) {
            result.push(this[i])
        }
    }
    return result
}
const b = [1, 3, 4, 5, 6, 2, 5, 1, 8, 20]
console.log(b._filter(item => item % 2 === 0)); // [ 4, 6, 2, 8, 20 ]

[dave-wind]

柯里化函数 累加函数

console.log(sum(1, 2)(3)()) // 6
console.log(sum(1)(2)(3)()) // 6
console.log(sum(1, 2, 4)(4)()) // 11

function sum(...args) {
    let params = args;

    const fn = (...newArgs) => {
       // 没有参数时候 直接执行
        if (newArgs.length == 0) {
            return params.reduce((pre, cur) => pre + cur, 0)
        } else {
            params = [...params, ...newArgs]; // 收集过程
            return fn
        }
    }
    return fn;
}
sum(1)(2)(3)() // 6