Right now we use codemirror's JavaScript mode on the website but there's a JSX mode now so we should start using that instead.

if (doc.cm && !doc.cm.curOp)
      return operation(doc.cm, makeChangeSingleDoc)(doc, change, selAfter, spans);
    if (change.to.line < doc.first) {
      shiftDoc(doc, change.text.length - 1 - (change.to.line - change.from.line));
      return;
    }
    if (change.from.line > doc.lastLine()) return;
    
    if (change.from.line < doc.first) {
      var shift = change.text.length - 1 - (doc.first - change.from.line);
      shiftDoc(doc, shift);
      change = {from: Pos(doc.first, 0), to: Pos(change.to.line + shift, change.to.ch),
                text: [lst(change.text)], origin: change.origin};
    }
    var last = doc.lastLine();
    if (change.to.line > last) {
      change = {from: change.from, to: Pos(last, getLine(doc, last).text.length),
                text: [change.text[0]], origin: change.origin};
    }
    change.removed = getBetween(doc, change.from, change.to);
    if (!selAfter) selAfter = computeSelAfterChange(doc, change, null);
    if (doc.cm) makeChangeSingleDocInEditor(doc.cm, change, spans, selAfter);
    else updateDoc(doc, change, spans, selAfter);
  }
  function makeChangeSingleDocInEditor(cm, change, spans, selAfter) {