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 (display.cachedCharWidth != null) return display.cachedCharWidth;
    var anchor = elt("span", "x");
    var pre = elt("pre", [anchor]);
    removeChildrenAndAdd(display.measure, pre);
    var width = anchor.offsetWidth;
    if (width > 2) display.cachedCharWidth = width;
    return width || 10;
  }
  
  
  
  
  
  var nextOpId = 0;
  function startOperation(cm) {