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Sometimes you'll want to create a shared module in an Angular app that defines services, pipes and directives that feature modules and lazy-loaded modules can use. The one little hiccup is that services, which are normally supposed to act as singletons, could end up being provided multiple times, especially for lazy-loaded modules. Luckily for us however, there's an easy fix for that specific use-case by defining a static <^>forRoot<^> method in the shared module that returns a <^>ModuleWithProviders<^> object.

Here's a sample implementation. First, our shared module:

				
					
[label ./shared/shared.module.ts]

import { NgModule, &lt;^&gt;ModuleWithProviders&lt;^&gt; } from '@angular/core';



import { MyDirective } from './my.directive';

import { FunPipe } from './fun.pipe';

import { SomeService } from './some.service';

Notice how we declare and export our pipes and directives as usual in the NgModule's metadata, but we don't provide the service. Instead, we define a static <^>forRoot<^> method in the module's class that returns an object that implements Angular's <^>ModuleWithProviders<^> interface.

Now, in our app module, we can import the shared module and call <^>forRoot<^> on it to also provide our service:

				
					
[label app.module.ts]

import { BrowserModule } from '@angular/platform-browser';

import { NgModule } from '@angular/core';



import { AppComponent } from './app.component';

&lt;^&gt;import { SharedModule } from './shared/shared.module'&lt;^&gt;;

You'll probably notice that you've seen this in action already when importing the <^>RouterModule<^> and calling forRoot on it in your app module.

Then finally, in any feature module we can simply import the shared module without the forRoot and we'll have access to the shared pipes and directives without providing the service again:

				
					
[label some-feature.module.ts]

import { NgModule } from '@angular/core';

import { CommonModule } from '@angular/common';



import { SharedModule } from '../shared/shared.module';

// ...

🍕 That's it! A neat little trick to make it easier to work with shared modules and lazy-loaded feature modules.