自己实现数组的 `map`、`filter`、`find` 方法

function map (arr, fun) {  let res = []  arr.forEach((...args) => {    res.push(fun(...args))  })  return res}function filter (arr, fun) {  let res = []  arr.forEach((...args) => {    if (fun(...args)) {      res.push(args[0])    }  })  return res}function find (arr, fun) {  let res = undefined  arr.some((...args) => {    if (fun(...args)) {      res = args[0]      return true    }    return false  })  return res}