SOURCE

 test (arr) {
      if(!Array.isArray(arr)) {
        arr = [...arr]
      }
      if(arr.length === 0) throw '不能为空'
      const num = arr[arr.length - 1]
      if(!isNaN(parseInt(num))) {
      return num;
      }
      return this.test(arr.slice(0, arr.length - 1))
    },