テキスト エディターでマルチカラー テキストの強調表示を実装するにはどうすればよいですか?

<code class="js">// Here's the JavaScript code
const keywords = ["var", "if", "else", "for"]; // Your list of keywords
const textarea = document.getElementById("textarea");

textarea.addEventListener("input", (e) => {
  const start = textarea.selectionStart;
  const end = textarea.selectionEnd;
  const text = textarea.value.substring(start, end);

  if (keywords.includes(text)) {
    // Apply highlighting to the selected text
    textarea.value = textarea.value.substring(0, start)
      + "<span class='highlight'>" + text + "</span>"
      + textarea.value.substring(end);
  }
});</code>

<code class="css">/* CSS for syntax highlighting*/
.highlight {
  color: red;
  font-weight: bold;
}</code>