Source: themes/ess-dive/loaders/optional.js

/**
 * RequireJS plugin
 *
 * optionally require a module to load.
 */
define([], function () {

    return {
        /**
         *
         * @param name - The name of the resource to load
         * @param parentRequire - A local "require" function to use to load other modules.
         * @param onload - A function to call with the value for name.
         */
        load: function(name, parentRequire, onload) {

            /**
             * This will load normally if the module is found. If the module fails
             * then {} will be returned
             */
            parentRequire([name], onload, function (err) {
                var failedModule = err.requireModules && err.requireModules[0];
                console.warn("Module " + failedModule+ " not loaded");
                requirejs.undef(failedModule);

                // Default returned is {}, if any module fails
                define(failedModule, [], function () {
                    return {};
                });

                // Call the parent methods
                parentRequire([failedModule], onload);
            });
    }
    };

});