How to implement custom code editor with HTML/JavaScript?

hello I'm trying to create syntax highlight in my web texteditor and my function is working and not at the same time I don't know why

when I type the word:

but in the editor:

this is the code:

const textarea = document.getElementById('editor');

updateText(textarea.value);

textarea.addEventListener("keydown", function(e) {
  setTimeout(() =>{
    updateText(textarea.value);
  },0)
})


function updateText(text)
{
  textarea.innerHTML = colorize(text.replace(/\n/g, "<br>").replace(/\t/g,"&#9;"));
}


function colorize(text)
{
var words = ["int","class","#include","namespace"];
  for(const keyword of words)
  {
    text = text.replaceAll(keyword,`<span style="color:#569cd6">${keyword}</span>`)
    text = text.replaceAll(keyword.toLowerCase(),`<span style="color:#569cd6">${keyword.toLowerCase()}</span>`)
  }
  return text
}

I tried to make it change color

Solution

So here's what I have tried following your approach, it still needs improvement, thou:

const textarea = document.getElementById("editor");

function moveCursorAtTheEnd() {
  var selection = document.getSelection();
  var range = document.createRange();
  var contenteditable = document.querySelector('div[contenteditable="true"]');
  if (contenteditable.lastChild.nodeType == 3) {
    range.setStart(contenteditable.lastChild, contenteditable.lastChild.length);
  } else {
    range.setStart(contenteditable, contenteditable.childNodes.length);
  }
  selection.removeAllRanges();
  selection.addRange(range);
}

textarea.addEventListener("keydown", function (e) {
  if (e.keyCode == 32 || e.keyCode == 13) updateText(textarea.textContent);
  textarea.innerHTML = textarea.innerHTML + " ";
  moveCursorAtTheEnd();
});

function updateText(text) {
  textarea.innerHTML = colorize(
    text.replace(/\n/g, "<br>").replace(/\t/g, "&#9;")
  );
}

function colorize(text) {
  var words = ["int", "class", "#include", "namespace"];
  for (const keyword of words) {
    text = text.replaceAll(
      keyword,
      `<span style="color:#569cd6">${keyword}</span>`
    );
    text = text.replaceAll(
      keyword.toLowerCase(),
      `<span style="color:#569cd6">${keyword.toLowerCase()}</span>`
    );
  }
  return text;
}

#editor {
  background: lightgrey;
  height: 100vh;
}
[contenteditable]:focus {
  outline: 0px solid transparent;
}

<div contentEditable="true" id="editor" onfocus="this.value = this.value;"></div>

After all, for a robust editor, I'd recommend you to use ACE editor, you can have a look at the live demo here: https://ace.c9.io/build/kitchen-sink.html It's not that difficult to implement. Hope it helps!