HTML側のインデントを抜き去れるinput

gistfile1.txt

<script>
  let indent = 0;

  function redraw() {
    const dom = document.querySelector('span');
    dom.innerHTML = '　'.repeat(indent);
  }
  function handle() {
    const dom = document.querySelector('input');
    if(dom.selectionStart === 0) {
      if(this.event.keyCode == 8) {
        indent = Math.max(0, indent - 1);
        redraw();
        return false;
      }
      if(this.event.keyCode == 32) {
        indent += 1;
        redraw();
        return false;
      }
    }
    return true;
  }
</script>
<span></span>・<input autofocus onkeydown="return handle()"></input>