function MaxSubStr(OriginalStr) {
      var max = 0
      var temArr
      for (let i = 0;i < OriginalStr.length - 1;i++) {
        temArr = []
        temArr.push(OriginalStr[i])
        for (let j = i + 1;j < OriginalStr.length;j++) {
          if (temArr.indexOf(OriginalStr[j]) < 0) {
            temArr.push(OriginalStr[j])
          } else if (temArr.indexOf(OriginalStr[j]) >= 0 && max < temArr.length) {
            max = temArr.length
            break
          }
        }
      }
      return max
    }

    var t = MaxSubStr('afashuih')
    console.log(t)
    /* 输入："afashuih"
    输出：6
    最长字串为："fashui" */

输出如下

算法复杂度比较高