RequireJS: Local fallback for if CDN fails

Question

In my backbone app, I need to provide a fallback for each required file, in the case that the CDN that delivers them fails.

I have tried overwriting require.onError like so:

require.onError = function (err) {
    if (err.requireType === 'timeout') {
        var url = err.requireModules;

        if (!!~url.indexOf("jquery/"))
            console.warn("CDN timed out, falling back to local jQuery.js")
            require(["libs/jquery"]);
            return;
        if (!!~url.indexOf("jqueryui/"))
            console.warn("CDN timed out, falling back to local jQueryUI.js")
            require(["libs/jqueryui"]);
            return;
        if (!!~url.indexOf("underscore"))
            console.warn("CDN timed out, falling back to local underscore.js")
            require(["libs/underscore"]);
            return;
        if (!!~url.indexOf("backbone"))
            console.warn("CDN timed out, falling back to local backbone.js")
            require(["libs/backbone"]);
            return;
    }
}

The problem is that this will asynchronously load the fallback files. I need these files to load in order, just like the original require statement, where I use the order! plugin.

With the overridden onError: when the CDN fails to load, the fallback load is started, but not waited for. This presents a problem because the scripts are ordered to be loaded based on their dependencies. Here is a look at my original require statement, that depends on the CDN:

require([
    "order!http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js",
    "order!http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.23/jquery-ui.min.js",
    "order!http://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.3.3/underscore-min.js",
    "order!http://cdnjs.cloudflare.com/ajax/libs/backbone.js/0.9.2/backbone-min.js",
    "order!utils/date",
    "order!core/core",
    "order!core/errors",
    "order!core/constants"
], function() {
    ...
}

Answer 1

What version of RequireJS are you using? It seems you might want to configure jQuery's fallback as a path in your config, then set up jQuery as a dependency on the other modules that need it. If you're using ~> 2.0, something like (untested):

// in your requirejs config
requirejs.config({
    //To get timely, correct error triggers in IE, force a define/shim exports 
    // check.
    enforceDefine: true,
    paths: {
        jquery: [
            'http://somecdn.com/jquery.min', // your cdn
            'lib/jquery' // your fallback
        ],
        jqueryui: "http://somecdn.com/jquery-ui.min.js"
    },
    shim: {
      jqueryui: ['jquery']
    }
});

// then in your requires
require([jquery, jqueryui, foo, bar], function($) {
    // stuff
});

They talk about how to do it in the wiki. If you are not using v2.x, there is a method for handling that here too.

If all the modules are configured to specify their own dependencies, you shouldn't need to worry about the order! directives either.

Answer 2

I have found a solution to the problem provided in RequireJS 2.x.x. There was a demand for this solution, so in turn, RequireJS added a paths object to their config. This provides fallback functionality for CDNs, should they fail.

It should also be noted that the order! plugin has been deprecated in Require 2.0, so I also needed to make use of the shim object to define dependencies. It's actually a pretty interesting idea.

Here is my new require.config:

require.config({
    urlArgs: "ts="+new Date().getTime(), // disable caching - remove in production
    paths: {
        jquery: [
            "http://ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min",
            "libs/jquery"
        ],
        jqueryui: [
            "http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.23/jquery-ui.min",
            "libs/jqueryui"
        ],
        underscore: [
            "http://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.3.3/underscore-min",
            "libs/underscore"
        ],
        backbone: [
            "http://cdnjs.cloudflare.com/ajax/libs/backbone.js/0.9.2/backbone-min",
            "libs/backbone"
        ]
    },
    shim: {
        'jqueryui': ['jquery'],
        'underscore': ['jquery'],
        'backbone': ['underscore'],
        'core/core': ['underscore'],
        'core/errors': ['core/core'],
        'core/constants': ['core/core']
    }
});