Monkey patches Express's res.render() to support a layout

gistfile1.js

/*
   Usage:
     Set a global/default layout with:
        app.set('view layout', 'foo');
     Set a layout per-render (overrides global layout) with:
        res.render('foo', { layout: 'bar' });
     Or disable a layout if a global layout is set with:
        res.render('foo', { layout: false });
     If no layout is provided using either of the above methods,
     then the view will be rendered as-is like normal.

     Inside your layout, the variable `body` holds the rendered partial/child view.

   Installation:
     Call `mpResponse();` before doing `require('express');` in your application.
*/

function mpResponse() {
  var expressResponse = require('express/lib/response'),
      expressResRender = expressResponse.render;
  expressResponse.render = function(view, options, fn) {
    options = options || {};
    var self = this,
        req = this.req,
        app = req.app,
        layout,
        cb;

    // support callback function as second arg
    if (typeof options === 'function')
      fn = options, options = {};

    // merge res.locals
    options._locals = self.locals;

    // default callback to respond
    fn = fn || function(err, str) {
      if (err) return req.next(err);
      self.send(str);
    };

    if (typeof options.layout === 'string')
      layout = options.layout;
    else if (options.layout !== false
             && typeof app.get('view layout') === 'string')
      layout = app.get('view layout');

    if (layout) {
      cb = function(err, str) {
        if (err) return req.next(err);
        options.body = str;
        expressResRender.call(self, layout, options, fn);
      };
    } else
      cb = fn;

    // render
    app.render(view, options, cb);
  };
}