Making the text bold bugs it and sometimes it switches on and off

I know the title wasn't very good but I have a div that has contenteditable="true" basically when CTRL + B is pressed the text you type should go bold and it works. But when I toggle it on it sometimes switches the bold on and off here is a snippet for it:

<html>
<head>
 <style>
 .textbox {
  border: 1px solid;
  border-radius: 5px;
  width: 50%;
  height: 170px;
  outnline: none;
  }
 .textbox: focus {
  border: 2px solid;
 } 
</style>
</head>
 <body>
 <div onkeyup="boldText()" contenteditable="true" class="textbox">
 </div>
<script>
function boldText() {
 if (event.keyCode == 17 + 66) {
  document.execCommand('bold');
 }
}
</script>
</body>
</html>

Here is a JSFiddle: https://jsfiddle.net/YT_Xaos/cogrm9b0

Solution

Well, first, document.execCommand(); is deprecated. I would try this.

function boldText(e) {
  if (e.keyCode == 17 && e.keyCode == 66) {
   let textBox = document.querySelector('.textbox');
   textBox.style.fontWeight = 'bold';
  }
}

document.addEventListener('keyup', boldText);

.textbox {
  border: 1px solid;
  border-radius: 5px;
  width: 50%;
  height: 170px;
  outline: none;
}
.textbox: focus {
  border: 2px solid;
}

<html>
 <body>
 <div contenteditable="true" class="textbox">
 </div>
</body>
</html>