Don't use transitive dependencies in AngularJS modules

Thomas Uhrig · November 11, 2016

Whenever you write an AngularJS application, you will use modules. Modules divide your application into small logical units. Some units might depend on others, while some might be totally independent.

angular.module('UserModule',[]); // no dependency!
angular.module('UtilModule',[]); // no dependency!
angular.module('MainModule',[ // 2 dependencies!

In the example above, you see three modules. A module called MainModule depending on two other modules, called UserModule and UtilModule. Note that this is just a very simple example. In the real world, things become much more complicated.

Let’s assume the following dependency: UserModule depends on some service from UtilModule. However, we forgot to add this dependency to the module declaration. But because we always run our MainModule for our application, everything will work perfectly fine. MainModule will load the dependency which the other module will need. So we won’t see this hidden transitive dependency easily.

However, we have problem now: we don’t know anymore which dependencies our module has. Even worse, our module depends on another module, because it provides its dependencies. We end up with a cyclic dependency as MainModule depends on UserModule and vice versa. One cannot run without the other.

But what’s the problem?

It becomes a practical problem when it comes down to testing. Whenever you do unit testing, you want to test things isolated:

beforeEach(function() {

You want to test a small piece with as less dependencies as possible. But what happens if you didn’t specify your dependencies right?

beforeEach(function() {

You also need to load modules which you don’t actually need. This could easily break your test if something in the other module changes.

Keep your modules clean!

Best regards, Thomas.