es5实现map/filter

// ES5循环循环实现filter

const selfFilter = function (fn, context) {      let arr = Array.prototype.slice.call(this)      let filteredArr = []      for (let i = 0; i < arr.length; i++) {          if(!arr.hasOwnProperty(i)) continue;           fn.call(context, arr[i], i, this) && filteredArr.push(arr[i])      }      return filteredArr  }

// ES5循环实现map

const selfMap = function (fn, context) {      let arr = Array.prototype.slice.call(this)      let mappedArr = []      for (let i = 0; i < arr.length; i++) {          // 判断稀疏数组的情况          if (!arr.hasOwnProperty(i)) continue;          mappedArr.push(fn.call(context, arr[i], i, this))      }      return mappedArr  }

// reduce实现map

const selfMap2 = function (fn, context) {      let arr = Array.prototype.slice.call(this)      return arr.reduce((pre, cur, index) => {          return [...pre, fn.call(context, cur, index, this)]      }, [])  }