Angular modules are fundamental components that serve as containers for your application’s functionality. They group together related code, making your app more organized and manageable. Each module can encapsulate various elements such as components, directives, services, and routing configurations, which helps in maintaining a modular architecture. For instance, the root module, often named AppModule, bootstraps the application, while feature modules can encapsulate specific features or functionalities, facilitating easier navigation and scalability.

To create a robust application, understanding how to define and use modules is essential. Here are some key aspects to consider when working with Angular modules:

1. NgModule decorator: Every Angular module is defined with the @NgModule decorator, which specifies the components, services, and other modules that the current module depends on.
2. Lazy loading: Modules can be loaded on demand, optimizing the application's performance and speeding up load times.
3. Shared modules: Frequently used components and services can be grouped in shared modules and imported into feature modules to promote reusability.

Maintaining Angular applications requires a strategic approach to ensure their performance and scalability. One of the **top practices** is to implement efficient component architecture. By using modular design principles, developers can break down large applications into smaller, reusable components. This promotes code reusability and makes it easier to manage dependencies. Additionally, it is crucial to keep components focused on a single responsibility, which enhances maintainability.

Another best practice is to regularly update dependencies and Angular versions. Keeping your application up-to-date not only ensures access to the latest features but also enhances security and performance. Utilize tools like Angular CLI to automate the update process and resolve any compatibility issues swiftly. Furthermore, maintaining thorough documentation throughout the development process is essential. Well-documented code and processes facilitate easier onboarding for new team members and ensure that knowledge is preserved, even as team dynamics change.

Angular is a powerful framework for building dynamic web applications, but developers often encounter a range of common errors that can hinder productivity. One frequent issue is the ‘Cannot read property of undefined’ error, which typically arises when attempting to access a property on an object that hasn’t been properly initialized. To fix this, ensure that the object is defined or use optional chaining (e.g., object?.property) to avoid runtime errors. Additionally, utilizing Angular’s built-in ngIf directive can help by conditionally rendering elements based on whether the object is defined.

Another common error developers face is ‘Expression has changed after it was checked’, which occurs when the state of a component changes after Angular has already checked a binding. This can lead to inconsistencies in the UI and can often be resolved by using the ChangeDetectorRef service. Specifically, calling this.changeDetectorRef.detectChanges() within the appropriate lifecycle hook such as ngAfterViewInit can help Angular re-evaluate the bindings properly. Remember, it's essential to manage the timing of state changes to prevent such errors in the first place.