JS Longest Palindrome Substring (Exhaustive)

JavaScript resolve longest palindrome substring problem by exhaustive.

by Tinytsunami

HTML

<div id="demo">
  String = <input type="text" /> <button>Process</button>
  <pre>waiting for you input something...</pre>
</div>

CSS

body {
  color: #ffffff;
  background: #20262e;
  font-family: monospace, sans-serif;
}

#demo {
  width: 400px;
  padding: 5px;
}

#demo input {
  color: #ffffff;
  background: #20262e;
  outline: none;
  border: none;
  border-bottom: 1px solid #ffffff;
}

#demo pre {
  width: 400px;
  height: 100px;
  border: solid 1px #ffffff;
  overflow-y: scroll;
}

#demo button {
  color: #ffffff;
  background: #20262e;
  border: 1px solid #ffffff;
  outline: none;
}

#demo button:hover {
  color: #20262e;
  background: #ffffff;
  border: 1px solid #ffffff;
}

JavaScript

(function() {
  /* get elements */
  let root = document.getElementById("demo");
  let inputNode = root.getElementsByTagName("input")[0];
  let buttonNode = root.getElementsByTagName("button")[0];
  let outputNode = root.getElementsByTagName("pre")[0];

  /* check is palindrome */
  let checkPalindrome = function(text, a, b) {
    let len = b - a + 1;
    let half = parseInt(len / 2);
    for (let i = 0; i < half; i++)
      if (text[a + i] != text[b - i])
        return false;
    return true;
  };

  /* evaluation time for functions */
  let evaluationTime = function(f, callback) {
    let start = Date.now();
    let value = f();
    callback.call(this, value, (Date.now() - start));
  };

  /* get LPS by exhaustive */
  let main = function(text) {
    let a, b, tmp, ans = "";
    for (a = 0; a < text.length; a++)
      for (b = a; b < text.length; b++)
        if (checkPalindrome(text, a, b)) {
          tmp = text.substring(a, b + 1);
          if (ans.length < tmp.length)
            ans = tmp;
        }
    return ans;
  };

  /* user input and process*/
  buttonNode.onclick = function() {
    evaluationTime(function() {
      return main(inputNode.value);
    }, function(LPS, time) {
      if (LPS.length > 30)
        LPS = `${LPS.substr(0, 30)}...`;
      outputNode.innerHTML += `\nLPS = "${LPS}"\nTime = ${time}ms\n`;
      outputNode.scrollTop = outputNode.scrollHeight;
    });
  };
})();