Angular Modules Standards · NG-03

Include a SharedModule for application-wide pipes, directives etc.

The SharedModule should also contain CommonModule and either FormsModule (from @angular/common) or ReactiveFormsModule (from @angular/forms).

Feature modules must be placed in a folder that is the same name as the feature it is for, eg users.

Feature modules must be used and should have their own routing module, to use lazy loading easily.

In addition to a feature module, if there is anything from a feature area that needs to be used by other module, use a FeatureSharedModule.

Include a CoreModule that contains all single use classes (interceptors/etc). This should be imported in your AppModule only.

It separates this logic away from your AppModule.

Any single use components (for example, navigation) should be declared in the CoreModule, not the AppModule. This is the whole purpose of the CoreModule, offsetting from the AppModule.

In most cases you should use providedIn: ‘root’ for services instead of adding them to the providers array of a Module.

This means that you don’t have unnecessary imports. providedIn: ‘root’ makes the service available to the whole app anyway.

If the service is only needed by a certain feature module then you should not use providedIn: ‘root’ and should instead add the service to the provider array for that feature module.

Re-importing of single use modules must be prevented.

Modules such as the CoreModule must only be imported once, importing them any more than that will cause problems.

...

@NgModule({...})
export class CoreModule {
    constructor(
        @Optional()
        @SkipSelf()
        core?: CoreModule
    ) {
        if (core) {
            throw new Error('Core injected more than once');
        }
    }
}