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.

for (var i = 0; i < spans.length; ++i) {
      var span = spans[i];
      if (span.from != null && span.from == span.to && span.marker.clearWhenEmpty !== false)
        spans.splice(i--, 1);
    }
    if (!spans.length) return null;
    return spans;
  }
  function mergeOldSpans(doc, change) {
    var old = getOldSpans(doc, change);
    var stretched = stretchSpansOverChange(doc, change);
    if (!old) return stretched;
    if (!stretched) return old;
    for (var i = 0; i < old.length; ++i) {
      var oldCur = old[i], stretchCur = stretched[i];
      if (oldCur && stretchCur) {
        spans: for (var j = 0; j < stretchCur.length; ++j) {
          var span = stretchCur[j];
          for (var k = 0; k < oldCur.length; ++k)
            if (oldCur[k].marker == span.marker) continue spans;
          oldCur.push(span);
        }
      } else if (stretchCur) {
        old[i] = stretchCur;
      }
    }
    return old;
  }
  function removeReadOnlyRanges(doc, from, to) {