Middleware for express/similar that adds a primitive 'res.yield' function based on 'res.render' but supports layouts by design. Note: clean-up pending.

express-yield.js

// Given 'layout.ejs' in your view folder, use `res.yield` like you would `res.render`.
// Override what layout to use by supplying a second string parameter right after the view.
// Layouts are passed `yield` as a local; in ejs, example usage: `<%- yield %>`.
//
// TODO: clean-up argument coercion
// TODO: better/cleaner documentation for usage of `yield`.
// TODO: extract functionality into a middleware-generating module to supply e.g. a default layout.
// TODO: (possibly) turn the layout's `yield` into a function to support nested yields.
app.use(function (req, res, next) {
  // default layout?
  var defaultLayout = 'layout.ejs';
  
  res.yield = function (view, layout, locals, fn) {
    console.log('attempting to yield');
    
    // handle 1 argument (just the view)
    if (typeof layout == 'undefined') {
      layout = defaultLayout;
      locals = {};
    }
    
    // support: layout and locals are optional.
    // handle 2 arguments
    if (typeof locals == 'undefined' && typeof layout == 'function') {
      fn = layout;
      locals = {};
      layout = defaultLayout;
    } else if (typeof locals == 'undefined' && typeof layout != 'string') {
      locals = layout;
      layout = defaultLayout;
    } else if (typeof locals == 'undefined') {
      locals = {};
    } 
    
    // handle 3 argument
    if (typeof fn == 'undefined' && typeof locals == 'function' && typeof layout != 'string') {
      fn = locals;
      locals = layout;
      layout = defaultLayout;
    } else if (typeof fn == 'undefined' && typeof locals == 'function') {
      fn = locals;
      locals = {};
    }
    
    // default callback if none provided
    var fn = typeof fn == 'function' ? fn : function(err, html) {
      if (err) {
        return this.next(err);
      }
      return this.send(html);
    };
    
    this.app.render(view, locals, (function(err, html) {
      if (err) return fn(err);
      
      locals.yield = html;
      this.app.render(layout, locals, fn.bind(this));
    }).bind(this));
  };
  
  next();
});