最长回文字符串

var longestPalindrome = function(s) {
  let start = (new Date()).valueOf()
  let arr = s.split('')
  let length = s.length
  let num = 0
  let str = ''
  function ishw(str){
    let ishw = true
    for(let i = 0; i < Math.floor(str.length/2); i++){
      if(str[i]!==str[str.length-i-1]){
        ishw = false
        break
      }
    }
    return ishw
  }
      for(let index in arr){
        for(let i = index;i<length;i++){
            let arr1 = s.slice(index,i+1)
            if(ishw(arr1)){
                if(arr1.length>num){
                    num = arr1.length
                    str = arr1
                }
            }
        }
      }

  console.log((new Date()).valueOf()-start)
  return str
};