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.

doc.history.undoDepth = this.history.undoDepth;
        doc.setHistory(this.getHistory());
      }
      return doc;
    },
    linkedDoc: function(options) {
      if (!options) options = {};
      var from = this.first, to = this.first + this.size;
      if (options.from != null && options.from > from) from = options.from;
      if (options.to != null && options.to < to) to = options.to;
      var copy = new Doc(getLines(this, from, to), options.mode || this.modeOption, from);
      if (options.sharedHist) copy.history = this.history;
      (this.linked || (this.linked = [])).push({doc: copy, sharedHist: options.sharedHist});
      copy.linked = [{doc: this, isParent: true, sharedHist: options.sharedHist}];
      return copy;
    },
    unlinkDoc: function(other) {
      if (other instanceof CodeMirror) other = other.doc;
      if (this.linked) for (var i = 0; i < this.linked.length; ++i) {
        var link = this.linked[i];
        if (link.doc != other) continue;
        this.linked.splice(i, 1);
        other.unlinkDoc(this);
        break;
      }
      
      if (other.history == this.history) {
        var splitIds = [other.id];
        linkedDocs(other, function(doc) {splitIds.push(doc.id);}, true);
        other.history = makeHistory();