Get username in javascript

I have a javscript the function of it is to search for text.
But instead of text i would like to it to search for logged in users username.

Can someone help me out?

<script>
var match = '<GET_LOGGED_USERNAME>',
    firstSibling = null,
    secondSibling = null;
 
window.onload = function(){
  var selector = document.querySelectorAll('td');
  for (i = 0; i < selector.length; ++i){
    if(selector[i].innerHTML == match){
      firstSibling = selector[i+1];
      break;
    }
  }
  
document.write(firstSibling.innerHTML);
}
</script>