手写数组中常用的ES6方法

    // 手写Array.map方法
    Array.prototype.map2 = function(callback, args) {
      if(typeof callback !== 'function') {
        throw new TypeError('callback is not a function')
      }
      let oldArr = Object.values(this)
      // 此时的this指向调用该方法的数组的对象
      let len = oldArr.length;
      let i = 0, res = []
      while(i < len) {
        res[i] = callback.call(this, oldArr[i])
        i++;
      }
      return res
    }

    // 手写Array.filter方法
    Array.prototype.filter2 = function(callback, args) {
      if(typeof callback !== 'function') {
        throw new TypeError('callback is not a function')
      }
      let oldArr = Object(this)
      let k = 0, res = [], len = oldArr.length
      while (k < len) {
        callback.call(this, oldArr[k]) && (res.push(oldArr[k]));
        k++
      }
      return res
    }

    // 手写Array.find方法
    Array.prototype.find2 = function(callback, args) {
      if(typeof callback !== 'function') {
        throw new TypeError('callback is not a function')
      }
      let oldArr = Object(this)
      let k = 0, len = oldArr.length, res
      while (k < len) {
        if( callback.call(this, oldArr[k])) { 
          res = oldArr[k];
          break;
        }               
        k++
      }
      return res
    }

    // 手写Array.some方法，判断是否存在满足条件的元素
    Array.prototype.some2 = function(callback, args) {
      if(typeof callback !== 'function') {
        throw new TypeError('callback is not a function')
      }
      let oldArr = Object(this)
      let k = 0, len = oldArr.length, res = false
      while (k < len) {
        if( callback.call(this, oldArr[k])) { 
          res = true;
          break;
        }               
        k++
      }
      return res
    }

    // 手写Array.reduce方法
    Array.prototype.reduce2 = function(callback, initVal) {
      if(typeof callback !== 'function') {
        throw new TypeError('callback is not a function')
      }
      let oldArr = Object(this)
      let k = 0, len = oldArr.length, res;
      if(arguments.length > 1 ) {
        // 存在初始值，
        res = initVal
      } else {
        // 无初始值，则初始为数组的第一个元素
        if(!oldArr.length) {
          throw new TypeError('Reduce of empty array with no initial value')
        }
        res = oldArr[0]
        k++   // 数组下标从1开始
      }
      while (k < len) {
        res = callback(res, oldArr[k], k, oldArr)         
        k++
      }
      return res
    }


    var arr = [{id: 1, name: '张三'},{id: 2,name:'李四'}, {id: 1, name: '王二'}]
    // var arr2 = arr.map2(function(ele) {
    //   return {
    //     id2: ele.id,
    //     name2: ele.name
    //   }
    // })
    // console.log(arr2);
    // var arr3 = arr.filter2(ele => ele.id === 5)
    // console.log(arr3);
    // var arr4 = arr.find2(ele => ele.id === 5)
    // console.log(arr4);
    // var arr5 = arr.some2(ele => ele.id === 6)
    // console.log(arr5);

    var nums = [1,2,3,4,5]
    var sum = nums.reduce2(function(pre,cur,index,arr) {
      return pre + cur;
    }, 100)
    console.log(sum);