Answer 1

 let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
      function reduction(arr) {
        let newArr = [...arr],res = []
        while (newArr.length) res.push(!(newArr.length % 2) ? newArr.shift() : newArr.pop())
        return res.concat(newArr).reverse()
      }
      let res = reduction(arr)
      console.log(res)

Answer 2

@bosens-China 我的理解应该是用于判断浏览器兼容性

Answer 3

小菜来了，膜拜maya大神