Modules that asynchronously require dependencies are often awkward to write. Dependencies that are only potentially loaded (perhaps based on user action, or non-deterministic input) usually end up inside of a nested, or deeply buried callback somewhere. This works, but kind of makes the dependency tree ugly, and isn't always obvious at a glance. It'd be great to be able to declare a top-level dependency with all of your other dependencies, but allow or force its resolution to be deferred. Enter dfdep.

Written for AMD or Browserify modules, dfdep allows you to declare a dependency that immediately returns a function, but allows you to invoke that function to retrieve the module when you actually want it. This is still an asynchronous action (via callbacks or promises), but helps unify the way that you declare and think about dependencies. It implements the browser side code to allow you to load in separate module bundles for different sections, without requiring nested require wrappers or one-off hacks.

It's much more of a pattern than a library, so if it seems like you're missing something, you probably aren't. We all know you're smart, you're just over-thinking it. They're async AND deferred dependencies, instead of solely async dependencies.

One advanced feature of deferred dependencies in the browser is time or activity based pre-caching.

You can allow dfdep to load your deferred dependency at any time without executing it.That could mean right away (weird), or perhaps just after the onload event, perhaps based on reduced user activity (implying you have a moment to increase network activity). Whatever the case, dfdep will give you a way to retrieve that module, and if it had already been preloaded and cached it will be much faster.

You can configure dfdep to fire at a certain time after onload, or you can specifically trigger a pre-cache of a dependency at any point prior to actually invoking the deferred function via a call to the precache function.

define(['a', 'dfd!b'], function (A, b) {
  var myA = A();

  // With a promise
  setTimeout(function() {
    b().then(function(b) {
      // Use b as you normally would
      myA.list.push(b.prop);
    });
  }, 10000);
});