手写实现数组方法

一、map

1.作用：对每个元素加工再返回新集合

2.代码实现：

            function myMap(fn, thisArg) {    
        if(typeof fn !== 'function') throw new Error('callback must be a function')
        if(typeof thisArg == undefined) throw new Error('thisArg must not be null or undefined')
        let originArr = this,
            len = originArr.length;
        let result = [];
        for (var i = 0;i < len;i++) {
          if(i in originArr){
            let singleResult = fn.call(thisArg, originArr[i],i,originArr);
              result.push(singleResult);
          }
        }
        return result;
      }
      Array.prototype.myMap = myMap;
      let arr = [1, 2, 3];
      console.log(
        arr.myMap(
          function(item) {
            return item * 2;
          },
          { name: "name" }
        )
      );

3.失误：

(1)遍历数组时用for of

不能把当前元素索引传入回调函数

(2) 忘记给回调函数传入第三个参数-遍历数组本身

(3)没有检查callback和thisArg参数

二、forEach

1.用法

循环遍历数组并将元素传入回调函数然后执行

2.代码实现

function myForEach(fn,thisArg){
    let originArr = this,
    len = originArr.length;
  for(var i = 0;i<len;i++){
    if(i in originArr){
        fn.call(thisArg,originArr[i],i,originArr)
    }
  }
}
Array.prototype.myForEach = myForEach
let arr = [1,2,3];
arr.myForEach(function(item,index){
    console.log(item,index,'iterator')
},{name:'my thisArg'})

三、filter

1.用法

遍历数组，将元素传入回调函数并执行，返回符合条件的元素合集

2.代码实现

      function myFilter(callback, thisArg) {
        if (typeof callback !== "function") {
          throw new Error("callback must be a function");
        }
        if (typeof thisArg == null) {
          throw new Error("thisArg must not be null or undifined");
        }
        let originArr = this,
          len = this.length;
        let result = [];
        for (var i = 0; i < len; i++) {
          if(i in originArr){
            let passed = callback.call(thisArg, originArr[i], i, originArr);
              if (passed) result.push(originArr[i]);
          }
        }
        return result;
      }
      Array.prototype.myFilter = myFilter;
      let arr = [1, 2, 3];
      console.log(
        arr.myFilter(
          function(item) {
            return item > 1;
          },
          { name: "name" }
        )
      );

四、reduce

1.用法

遍历数组累计计算返回结果
参数：callback和initialValue

2.代码

function myReduce(callback,initialValue){
  if(typeof callback !== 'function') throw new Error('callback must be a function')
    let originArr = Object(this),
      k = 0,
      len = originArr.length;
  let accumulotar = initialValue;
  console.log(accumulotar)
  if(accumulotar === undefined){
      while(k < len && !(k in originArr)){
        k++
    }
    if(k >= len){
        throw new Error('Reduce of empty array with no initial value')
    }
    accumulotar = originArr[k++]
  }
    while(k < len){
    if(k in originArr){
      console.log(k)
        accumulotar = callback.call(undefined,accumulotar,originArr[k],k,originArr)
    }
      k++
  }
  return accumulotar
}
Array.prototype.myReduce = myReduce
let arr = new Array(2);
arr.push(1)
arr.push(2)
console.log(arr.myReduce((accumulotar,element)=>{return accumulotar + element},8))

五、Function.prototype.apply

1.用法

改变函数执行上下文this指向，借用函数(Object.prototype.toString, 构造函数借用)

2.代码

      function myApply(thisArg = window, ...args) {
        let originFunction = this;
        if (typeof originFunction !== "function") {
          throw new Error("type error");
        }
        let functionName = Symbol();
        thisArg[functionName] = originFunction;
        return thisArg[functionName](args);
      }
            Function.prototype.myApply = myApply

六、模拟new关键字

1.用法

（1）先一个新对象并继承构造函数原型
（2）借用构造函数给新对象添加属性
（3）根据构造函数中的返回值，来判断返回新对象，还是构造函数的返回值

2.代码

function myNew(constructor,...args){
    let instance = Object.create(constructor.prototye),
          result = constructor.apply(instance,args),
      resultType = typeof result;
  let isObject = resultType === 'object' && !== null,
      isFunction = resultType === 'function';
  return isObject || isFunction ? result : instance
}