Angular is a powerful framework for building dynamic web applications. However, like any tool, it can be misused, leading to inefficient code, bugs, and headaches. Here are some common Angular coding issues and how to fix them, we will delve deeper into more advanced techniques and best practices to further enhance your Angular development skills.

1. Misunderstanding Observables

Angular uses Observables extensively, but they can be tricky to understand. An Observable is a stream of values that you can subscribe to, like this:

this.http.get('/api/data').subscribe(data => console.log(data));

The problem is, if you forget to unsubscribe, you can end up with memory leaks and unexpected behavior.

Solution: Always unsubscribe from Observables when you’re done with them. You can do this manually, or use the `takeUntil` operator to automatically unsubscribe when a certain condition is met.

private destroyed$ = new Subject();

ngOnInit() {
 this.http.get('/api/data')
 .pipe(takeUntil(this.destroyed$))
 .subscribe(data => console.log(data));
}

ngOnDestroy() {
 this.destroyed$.next();
 this.destroyed$.complete();
}

2. Overusing Two-Way Data Binding

Two-way data binding is a powerful feature of Angular, but it can lead to performance issues if overused. Every time the model changes, Angular has to check all the bindings, which can slow down your app.

<input [(ngModel)]="name">

Solution: Use one-way data binding whenever possible, and only use two-way data binding when necessary.

<input [value]="name" (input)="name = $event.target.value">

3. Not Using Angular CLI

Angular CLI is a command-line interface for Angular. It can generate boilerplate code, run tests, and build your app. If you’re not using it, you’re missing out on a lot of productivity.

Solution: Start using Angular CLI. It’s as simple as installing it with npm and running `ng new app_name`.

4. Ignoring Error Handling

Many developers ignore error handling, especially when dealing with Observables. This can lead to unhandled exceptions and hard-to-debug issues.

this.http.get('/api/data').subscribe(data => console.log(data));

Solution: Always handle errors in your Observables. You can do this with the `catchError` operator.

this.http.get('/api/data')
 .pipe(catchError(err => {
   console.error(err);
   return of([]);
 }))
 .subscribe(data => console.log(data));

5. Not Using Async Pipe

The async pipe automatically subscribes and unsubscribes from Observables, reducing boilerplate and preventing memory leaks. If you’re not using it, you’re making your life harder than it needs to be.

Solution: Use the async pipe whenever possible.

<div *ngIf="data$ | async as data">{{ data }}</div>

6. Neglecting Unsubscription

Subscriptions to Observables can lead to memory leaks if not properly managed.

// Problematic code
ngOnInit() {
 this.subscription = this.myService.getData().subscribe(data => {
   // do something with data
 });
}

In the above example, we subscribe to an Observable but never unsubscribe, leading to potential memory leaks.

Solution: Always remember to unsubscribe from Observables in ngOnDestroy lifecycle hook.

// Corrected code
ngOnInit() {
 this.subscription = this.myService.getData().subscribe(data => {
   // do something with data
 });
}

ngOnDestroy() {
 if (this.subscription) {
   this.subscription.unsubscribe();
 }
}

7. Not Leveraging Lazy Loading

Lazy loading is a design pattern in Angular that allows you to load JavaScript components asynchronously when a specific route is activated. This can significantly improve performance by only loading parts of your app that are needed.

Solution: Use Angular’s built-in support for lazy loading. This involves creating a separate module for each feature and configuring your routes to load them asynchronously.

const routes: Routes = [
  {
    path: 'feature',
    loadChildren: () => import('./feature/feature.module').then(m => m.FeatureModule)
  }
];

8. Ignoring Change Detection Strategy

By default, Angular uses the ChangeDetectionStrategy.Default which checks for changes in every component tree whenever something changes. This can lead to performance issues in large applications.

Solution: Use ChangeDetectionStrategy.OnPush where possible. This tells Angular to only check for changes within the component if its input properties change.

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  changeDetection: ChangeDetectionStrategy.OnPush
})
export class MyComponent {}

9. Not Using TrackBy with *ngFor

When using *ngFor to loop over an array in templates, Angular keeps track of each element with an internal object ID. If the array changes, Angular re-renders the entire DOM tree. But if the objects have a unique id, this can be avoided.

Solution: Use trackBy function in *ngFor. This function takes the index and the current item and returns the unique id.

<div *ngFor="let item of items; trackBy: trackByFn">{{ item.name }}</div>

trackByFn(index, item) {
  return item.id; // unique id corresponding to the item
}

10. Not Using Content projection

Content projection (previously known as transclusion in AngularJS) allows you to create reusable components by injecting HTML from the outside.

Solution: Use <ng-content> to mark the place in a component’s template where the projected content should go.

<app-my-component>
  <div>This will be projected</div>
</app-my-component>

<!-- my-component.component.html -->
<div>
  <ng-content></ng-content>
</div>

11. Not Using TypeScript to Its Full Potential TypeScript

The language that Angular is written in, provides static types. This can help catch errors early, improve tooling, and make your code more self-documenting.

Solution: Use TypeScript’s advanced features like interfaces, type guards, and enums. Also, consider using TSLint to enforce coding standards.

interface User {
  name: string;
  age: number;
}

function isUser(user: any): user is User {
  return typeof user.name === 'string' && typeof user.age === 'number';
}

By avoiding these advanced Angular coding issues, you can write more efficient, maintainable, and robust code.

Happy coding!