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 (cm.doc.mode.startState && cm.doc.frontier < cm.display.showingTo)
      cm.state.highlight.set(time, bind(highlightWorker, cm));
  }
  function highlightWorker(cm) {
    var doc = cm.doc;
    if (doc.frontier < doc.first) doc.frontier = doc.first;
    if (doc.frontier >= cm.display.showingTo) return;
    var end = +new Date + cm.options.workTime;
    var state = copyState(doc.mode, getStateBefore(cm, doc.frontier));
    var changed = [], prevChange;
    doc.iter(doc.frontier, Math.min(doc.first + doc.size, cm.display.showingTo + 500), function(line) {
      if (doc.frontier >= cm.display.showingFrom) { 
        var oldStyles = line.styles;
        line.styles = highlightLine(cm, line, state, true);
        var ischange = !oldStyles || oldStyles.length != line.styles.length;
        for (var i = 0; !ischange && i < oldStyles.length; ++i) ischange = oldStyles[i] != line.styles[i];
        if (ischange) {
          if (prevChange && prevChange.end == doc.frontier) prevChange.end++;
          else changed.push(prevChange = {start: doc.frontier, end: doc.frontier + 1});
        }
        line.stateAfter = copyState(doc.mode, state);
      } else {
        processLine(cm, line.text, state);
        line.stateAfter = doc.frontier % 5 == 0 ? copyState(doc.mode, state) : null;
      }
      ++doc.frontier;
      if (+new Date > end) {
        startWorker(cm, cm.options.workDelay);