Handling HTTP errors in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Angular HTTP client module.

Angular

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [[1]]
})
export class AppModule {}

Drag options to blanks, or click blank then click option'

AHttpClientModule

BHttpModule

CHttpClient

DHttpErrorResponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using HttpModule which is deprecated.

Importing HttpClient instead of HttpClientModule.

Confusing HttpErrorResponse with the module.

You need to import HttpClientModule to use HTTP services in Angular.

2fill in blank

medium

Complete the code to inject HttpClient into the service constructor.

Angular

import { HttpClient } from '@angular/common/http';

@Injectable({ providedIn: 'root' })
export class DataService {
  constructor(private [1]: HttpClient) {}
}

Drag options to blanks, or click blank then click option'

AHttpErrorResponse

BHttpClientModule

Chttp

DhttpClient

Attempts:

3 left

💡 Hint

Common Mistakes

Using the module name instead of a variable name.

Using uppercase variable names.

Using the error class name as variable.

The injected variable name is usually http or similar, but must be a valid identifier.

3fill in blank

hard

Fix the error in the HTTP GET request to handle errors using catchError.

Angular

this.http.get('/api/data').pipe(
  [1](error => {
    console.error('Error occurred:', error);
    return throwError(() => error);
  })
).subscribe(data => console.log(data));

Drag options to blanks, or click blank then click option'

AcatchError

BcatchErrors

ChandleError

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'catch' which is not an RxJS operator.

Using a non-existent 'catchErrors'.

Using a custom function name instead of the operator.

The correct RxJS operator to catch errors is catchError.

4fill in blank

hard

Fill both blanks to create an error handling function that logs and rethrows the error.

Angular

import { throwError } from 'rxjs';
import { HttpErrorResponse } from '@angular/common/http';

handleError(error: [1]) {
  console.error('An error happened:', error.message);
  return [2](() => error);
}

Drag options to blanks, or click blank then click option'

AHttpErrorResponse

BthrowError

CcatchError

DError

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic Error instead of HttpErrorResponse.

Using catchError instead of throwError to rethrow.

Not returning an observable from the function.

The error parameter type is HttpErrorResponse and throwError is used to rethrow the error as an observable.

5fill in blank

hard

Fill all three blanks to use the error handler in the HTTP request with pipe and catchError.

Angular

this.http.get('/api/items').pipe(
  [1](this.[2]),
  [3](this.handleError)
).subscribe({
  next: data => console.log(data),
  error: err => console.log('Handled error:', err)
});

Drag options to blanks, or click blank then click option'

Atap

Bpipe

CcatchError

Dretry

Attempts:

3 left

💡 Hint

Common Mistakes

Using retry instead of pipe to chain operators.

Using pipe inside pipe.

Not using catchError to handle errors.

Use pipe to chain operators, tap for side effects, and catchError to handle errors.

Practice

1. What is the main purpose of using catchError in Angular HTTP requests?

easy

A. To handle errors that occur during the HTTP request

B. To retry the HTTP request automatically

C. To transform the HTTP response data

D. To cancel the HTTP request

Solution

1. Step 1: Understand the role of catchError

   catchError is used to catch errors in an observable stream, such as HTTP requests.

2. Step 2: Identify its purpose in HTTP requests

   It allows handling errors gracefully instead of letting them crash the app or go unhandled.

3. Final Answer:

   To handle errors that occur during the HTTP request -> Option A

4. Quick Check:

   catchError handles HTTP errors = A [OK]

Hint: Remember catchError catches errors in observable streams [OK]

Common Mistakes:

• Thinking catchError retries requests automatically
• Confusing catchError with data transformation operators
• Assuming catchError cancels requests

2. Which of the following is the correct syntax to catch HTTP errors in Angular using RxJS operators?

easy

A. this.http.get(url).catchError(error => throwError(error))

B. this.http.get(url).pipe(catchError(error => throwError(() => error)))

C. this.http.get(url).pipe(map(error => throwError(error)))

D. this.http.get(url).subscribe(catchError(error => throwError(error)))

Solution

1. Step 1: Recognize correct RxJS operator usage

   In Angular, pipe() is used to chain operators like catchError.

2. Step 2: Check the syntax of catchError

   The correct form is catchError(error => throwError(() => error)) inside pipe().

3. Final Answer:

   this.http.get(url).pipe(catchError(error => throwError(() => error))) -> Option B

4. Quick Check:

   Use pipe with catchError and throwError = B [OK]

Hint: Always use pipe() to chain catchError in Angular HTTP calls [OK]

Common Mistakes:

• Using catchError outside pipe()
• Using map instead of catchError for errors
• Passing throwError without a function wrapper

3. Consider this Angular service code snippet:

getData() {
  return this.http.get('/api/data').pipe(
    catchError(err => {
      console.error('Error:', err.message);
      return throwError(() => new Error('Failed to load data'));
    })
  );
}

What will happen when the HTTP request fails?

medium

A. The HTTP request is retried automatically

B. The error is ignored and the observable completes successfully

C. The error is logged and a new error is passed to subscribers

D. The app crashes due to unhandled error

Solution

1. Step 1: Analyze catchError behavior on failure

   When the HTTP request fails, catchError runs, logging the error message.

2. Step 2: Understand throwError usage

   It returns a new error observable with a friendly message, passing it to subscribers.

3. Final Answer:

   The error is logged and a new error is passed to subscribers -> Option C

4. Quick Check:

   catchError logs and rethrows error = A [OK]

Hint: catchError logs error then returns throwError to notify subscribers [OK]

Common Mistakes:

• Assuming error is ignored or swallowed
• Thinking HTTP request retries automatically
• Believing app crashes without catchError

4. Identify the error in this Angular HTTP error handling code:

this.http.get('/api/items').pipe(
  catchError(error => {
    console.log('Error:', error);
    return throwError(error);
  })
).subscribe(data => console.log(data));

medium

A. throwError should be called with a function: throwError(() => error)

B. catchError should be outside pipe()

C. subscribe should be before pipe()

D. console.log cannot be used inside catchError

Solution

1. Step 1: Check throwError usage

   In RxJS 7+, throwError requires a function returning the error: throwError(() => error).

2. Step 2: Verify catchError and subscribe placement

   catchError must be inside pipe(), and subscribe after pipe(), which is correct here.

3. Final Answer:

   throwError should be called with a function: throwError(() => error) -> Option A

4. Quick Check:

   throwError needs function wrapper = D [OK]

Hint: Use throwError(() => error) to avoid RxJS errors [OK]

Common Mistakes:

• Calling throwError directly with error object
• Placing catchError outside pipe()
• Misordering subscribe and pipe()

5. You want to show a friendly message to users when an HTTP error occurs, but still log the original error for debugging. Which code snippet correctly implements this in Angular?

hard

A. return this.http.get(url).pipe(catchError(err => { console.error(err); return of('Error occurred'); }))

B. return this.http.get(url).pipe(catchError(err => { return throwError(() => new Error('Error occurred')); }))

C. return this.http.get(url).pipe(catchError(err => { alert(err.message); return throwError(() => err); }))

D. return this.http.get(url).pipe(catchError(err => { alert('Error occurred'); console.error(err); return throwError(() => err); }))

Solution

1. Step 1: Check user notification and logging

   return this.http.get(url).pipe(catchError(err => { alert('Error occurred'); console.error(err); return throwError(() => err); })) shows an alert to users and logs the error to console, fulfilling both requirements.

2. Step 2: Verify error propagation

   It returns throwError(() => err) to pass the error downstream for further handling.

3. Final Answer:

   return this.http.get(url).pipe(catchError(err => { alert('Error occurred'); console.error(err); return throwError(() => err); })) -> Option D

4. Quick Check:

   Alert user + log error + rethrow = C [OK]

Hint: Alert user, log error, then rethrow with throwError(() => err) [OK]

Common Mistakes:

• Not alerting user or not logging error
• Returning a string instead of throwing error
• Alerting raw error message confusing users