How to get Backbone, Underscore (with your own extensions) and RequireJS to work together.

backbone-amd.js

// Use the version from Backbone AMD repository:
// https://github.com/amdjs/backbone

requirejs-config.js

require.config({

  paths:
	{
		underscore : 'libs/underscore-with-mixins',
		jquery     : 'libs/jquery',
		backbone   : 'libs/backbone-amd'
	},

	shim: {
	        'libs/underscore-original': {
	            exports: '_'
	        }
	}

});

underscore-original.js

// Use the original Underscore.js file

underscore-with-mixins.js

define([
  'libs/underscore-original'
], function (
	_
) {
  
  // Define your own Underscore mixins here
  
});