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.

var filter = hasHandler(doc, "beforeChange") || doc.cm && hasHandler(doc.cm, "beforeChange");
    for (var i = event.changes.length - 1; i >= 0; --i) {
      var change = event.changes[i];
      change.origin = type;
      if (filter && !filterChange(doc, change, false)) {
        (type == "undo" ? hist.done : hist.undone).length = 0;
        return;
      }
      anti.changes.push(historyChangeFromChange(doc, change));
      var after = i ? computeSelAfterChange(doc, change, null)
                    : {anchor: event.anchorBefore, head: event.headBefore};
      makeChangeSingleDoc(doc, change, after, mergeOldSpans(doc, change));
      var rebased = [];
      linkedDocs(doc, function(doc, sharedHist) {
        if (!sharedHist && indexOf(rebased, doc.history) == -1) {
          rebaseHist(doc.history, change);
          rebased.push(doc.history);
        }
        makeChangeSingleDoc(doc, change, null, mergeOldSpans(doc, change));
      });
    }
  }
  function shiftDoc(doc, distance) {
    function shiftPos(pos) {return Pos(pos.line + distance, pos.ch);}
    doc.first += distance;
    if (doc.cm) regChange(doc.cm, doc.first, doc.first, distance);
    doc.sel.head = shiftPos(doc.sel.head); doc.sel.anchor = shiftPos(doc.sel.anchor);
    doc.sel.from = shiftPos(doc.sel.from); doc.sel.to = shiftPos(doc.sel.to);
  }
  function makeChangeSingleDoc(doc, change, selAfter, spans) {