Should I use readonly on injected Angular services instead of making them public?

2 min read 06-10-2024

Should I use readonly on injected Angular services instead of making them public?

Angular Dependency Injection: readonly vs. Public - Which is Right for You?

In Angular, dependency injection is a powerful mechanism for managing the lifecycle of your components and services. While it's generally good practice to keep your services private, you might wonder if using readonly properties for injected services offers any advantages over simply making them public.

Let's explore the nuances of each approach, analyze their strengths and limitations, and help you decide which strategy best suits your needs.

The Scenario

Imagine you have a component that relies on a UserService for retrieving user information. You might inject this service into your component like this:

import { Component } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user-profile',
  template: `
    <!-- Display user data retrieved from the service -->
  `
})
export class UserProfileComponent {
  constructor(private userService: UserService) {}
}

Now, you can access the UserService within your component. But how should you expose it?

Public vs. Readonly

Public:

Exposure: The service is accessible directly from any part of your component.
Flexibility: You can freely call any method or property of the service.
Potential for misuse: Other parts of the component could potentially modify the service's state, leading to unexpected behavior.

Readonly:

Exposure: The service is accessible but only for reading its properties or methods.
Protection: Prevents accidental modification of the service's state.
Enhanced control: Ensures the service's integrity remains intact.

Choosing the Right Approach

When to use readonly:

Service encapsulation: You want to ensure the injected service's internal state remains untouched by your component.
Preventing accidental modifications: You want to avoid any unintended side effects from altering the service.
Maintaining data integrity: You need to guarantee the service's data remains consistent.

When to use public:

Deep integration: You require extensive access to the service's methods and properties, potentially for complex interactions or customizations.
Performance optimization: Direct access to the service might be necessary in specific cases to improve performance, especially if you need to frequently call the service.
Legacy code: You might be working with existing code that relies on public access.

Additional Considerations

TypeScript's readonly keyword: In TypeScript, the readonly keyword prevents assigning a new value to a property after initialization.
Angular's dependency injection: Angular injects dependencies at the component's constructor. readonly ensures that the injected service remains immutable throughout the component's lifecycle.
Best practices: While the choice between readonly and public depends on your specific use case, striving for a more controlled and predictable environment often leads to better code quality.

Conclusion

The decision between readonly and public for injected services in Angular depends on your specific needs and design goals. By understanding the implications of each approach, you can make informed choices that enhance your code's maintainability, reduce errors, and promote more robust applications.