JSFiddle - React, Tailwind, and code Playground

HTML

<pre>
<span>1</span>Test();
<span>2</span>$foo = 42;
</pre>

<pre class="autolines">
Test();
$foo = 42;
</pre>

CSS

span {
   -moz-user-select: -moz-none;
   -khtml-user-select: none;
   -webkit-user-select: none;
   -ms-user-select: none;
   user-select: none;
   min-width:2em;
   display:inline-block;
}

JavaScript

$(function () {
  $('.autolines').each (function () {
  var box  = $(this);
  var code = $.trim (box.text ());
  var lines = code.split(/\r?\n/);

  code = '';
  for (var index = 0; index < lines.length ; ++index) {
        code = code + '<span>' + index + '</span>' + lines[index] + '\n';
  }
  box.html (code);
  });
});