How do you build custom Angular libraries?

Angular libraries allow you to organize and reuse your code across multiple Angular applications or share it with other developers. They promote modularity, maintainability, and reusability. This guide outlines the steps to create and publish your own custom Angular library.

Why Build a Custom Angular Library?

Custom Angular libraries are ideal for encapsulating common components, services, directives, or pipes that can be shared across various projects. This approach helps in maintaining a consistent user experience, reducing code duplication, and simplifying project management, especially in large organizations with multiple applications.

Steps to Build a Custom Angular Library

Follow these steps to create, develop, and potentially publish your own Angular library.

1. Create a New Workspace (if needed)

If you don't already have an Angular workspace, create one. It's often recommended to create a workspace that doesn't include an initial application, as the primary purpose of this workspace will be to host your library.

ng new my-library-workspace --no-create-application

2. Generate the Library

Navigate into your workspace and use the Angular CLI to generate a new library. Replace my-awesome-library with your desired library name.

cd my-library-workspace
ng generate library my-awesome-library --prefix=mal

The --prefix flag is optional but recommended. It sets a unique prefix for your library's selectors and input/output properties to avoid naming conflicts. The CLI will create a projects/my-awesome-library directory and automatically configure angular.json.

3. Develop Your Library's Features

Inside projects/my-awesome-library/src/lib/, you'll find a basic component, service, and module. This is where you'll add all your custom components, services, directives, pipes, etc. You can generate new elements using the CLI:

cd projects/my-awesome-library
ng generate component my-custom-button
ng generate service data-sharing

Implement the logic and templates for your library's features as you would in a regular Angular application.

4. Export Library Assets via `public-api.ts`

The public-api.ts file (located at projects/my-awesome-library/src/public-api.ts) is crucial. It defines what parts of your library are exposed to consumers. Only items exported from this file will be available when someone imports your library.

/*
 * Public API Surface of my-awesome-library
 */

export * from './lib/my-awesome-library.service';
export * from './lib/my-awesome-library.component';
export * from './lib/my-custom-button/my-custom-button.component';
export * from './lib/data-sharing.service';
export * from './lib/my-awesome-library.module';

5. Build the Library

Before you can use or publish your library, you need to build it. The build process compiles your library and places it in the dist/ folder of your workspace.

ng build my-awesome-library --configuration production

The --configuration production flag ensures an optimized build. The compiled library will be located at dist/my-awesome-library.

6. Use the Library in Another Application

Once built, you can use your library in any Angular application. There are two main scenarios:

a) Within the same workspace (local testing): If your application is in the same workspace as the library, you can import it directly after building.

// In src/app/app.module.ts of your consuming application
import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { MyAwesomeLibraryModule } from 'my-awesome-library'; // Import by library name

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    MyAwesomeLibraryModule // Add to imports array
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

b) In an external application (after publishing): If you've published your library (e.g., to npm), you can install and use it like any other third-party package.

npm install my-awesome-library

// Then import and use it as shown above in app.module.ts

7. Publish the Library (Optional)

If you want to share your library publicly or with other internal projects that are not in the same workspace, you can publish it to npm (or a private npm registry).

cd dist/my-awesome-library
npm publish

Ensure you have an npm account and are logged in (npm login). Update the version in projects/my-awesome-library/package.json before publishing new versions.

Best Practices for Library Development

• Modularity: Keep your library focused on specific functionality. If it grows too large, consider splitting it into smaller, independent libraries.
• Documentation: Provide clear and comprehensive documentation for how to use your components, services, and modules, including examples.
• Testing: Write robust unit and integration tests for your library to ensure reliability and prevent regressions.
• Versioning: Follow Semantic Versioning (SemVer) (MAJOR.MINOR.PATCH) to communicate changes and breaking updates clearly.
• Peer Dependencies: Use peerDependencies in package.json to specify compatible versions of Angular and other common libraries your library depends on, rather than dependencies.
• Lazy Loading: If your library contains large modules, consider structuring it to support lazy loading in consuming applications.

By following these steps and best practices, you can effectively build and manage custom Angular libraries, significantly enhancing code reusability and project scalability.