index.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <meta http-equiv="X-UA-Compatible" content="ie=edge" />
  <title>Document</title>
</head>

<body>
  <input type="text" id="txt" />
  <button onclick="checking()">Check</button>
  <p id="p1"></p>
</body>
<script>
  function checking() {
    var val = "";
    var re;
    var lowRegStr;
    var reverseStr;
    var chk = "";
    var maxp_length = 0;
    var maxp = [];
    val = document.getElementById("txt").value;

    function palindrome(str) {
      re = /[\W_]/g;
      lowRegStr = str.toLowerCase().replace(re, "");
      reverseStr = lowRegStr
        .split("")
        .reverse()
        .join("");
      return reverseStr === lowRegStr;
    }

    chk = palindrome(val);
    if (chk == true) {
      document.getElementById("p1").innerHTML = `${val} is a palindrome word`;
    } else {
      longestPalind(val);
    }

    function longestPalind(s) {
      for (var i = 0; i < s.length; i++) {
        var subs = s.substr(i, s.length);
        for (var j = s.length; j >= 0; j--) {
          var sub_subs = subs.substr(0, j);
          if (palindrome(sub_subs) && sub_subs.length > 2) {
            if (sub_subs.length > maxp_length) {
              maxp_length = sub_subs.length;
              maxp.push(sub_subs);
            }
          }
        }
      }
    }
    console.log(maxp);
    if (maxp == []) {
      return "no palindrome";
    } else {
      var length = 0;
      var lpalind;
      for (var i = 0;i < maxp.length;i++) {
        if (maxp[i].length > length) {
          length = maxp[i].length;
          lpalind = maxp[i];
        }
      }
      console.log(`${lpalind} is the longest palindrome in this sentence`);
    }
  }
</script>

</html>