Handling HTTP errors in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when the HTTP request fails in this Angular service?

Consider this Angular service method that fetches user data. What will the component receive if the HTTP request returns a 404 error?

Angular

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class UserService {
  constructor(private http: HttpClient) {}

  getUser(id: number) {
    return this.http.get(`/api/users/${id}`).pipe(
      catchError(error => {
        return throwError(() => new Error('User not found'));
      })
    );
  }
}

A The component receives null.

B The component receives the original HTTP error object.

C The component receives an error with message 'User not found'.

D The component receives an empty object {}.

💡 Hint

Look at what the catchError operator returns inside the pipe.

The catchError operator intercepts the HTTP error and returns a new error with the message 'User not found'. So the component will receive this new error, not the original HTTP error.

📝 Syntax

intermediate

Which option correctly handles HTTP errors using Angular's HttpClient?

Choose the code snippet that correctly catches HTTP errors and returns a fallback value.

Angular

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { catchError, of } from 'rxjs';

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

  fetchData() {
    return this.http.get('/api/data').pipe(
      // error handling here
    );
  }
}

A catchError(error => of({ data: [] }))

B catchError(error => { return { data: [] }; })

C catchError(error => throwError(() => new Error('Error occurred')))

D catchError(error => console.log(error))

💡 Hint

Remember catchError expects an observable to be returned.

Option A returns an observable of a fallback object using of(). Option A returns a plain object, which is invalid. Option A rethrows an error, not a fallback value. Option A returns undefined, causing errors.

🔧 Debug

advanced

Why does this Angular HTTP error handler not catch errors?

This Angular service method tries to catch HTTP errors but errors still propagate unhandled. What is the problem?

Angular

getData() {
  this.http.get('/api/data').subscribe({
    next: data => console.log(data),
    error: err => console.error('Error:', err)
  });

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

A The first HTTP call subscribes without catchError, so errors are not caught there.

B The catchError operator is used incorrectly inside subscribe.

C The catchError operator must be placed after subscribe.

D The service returns two observables causing conflict.

💡 Hint

Check where the subscription happens and if catchError is applied before it.

The first HTTP call subscribes directly without catchError, so errors from that call are not caught. The second call has catchError but is not subscribed to here, so it does not affect the first call's errors.

❓ state_output

advanced

What is the component's errorMessage after this HTTP error handling?

In this Angular component, what will be the value of errorMessage after a 500 server error occurs?

Angular

import { Component } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

@Component({
  selector: 'app-data',
  template: `<p>{{ errorMessage }}</p>`
})
export class DataComponent {
  errorMessage = '';

  constructor(private http: HttpClient) {
    this.http.get('/api/data').pipe(
      catchError(err => {
        this.errorMessage = 'Server error occurred';
        return throwError(() => err);
      })
    ).subscribe({
      next: data => console.log(data),
      error: err => console.log('Error handled in subscribe')
    });
  }
}

A '' (empty string)

B 'Server error occurred'

C 'Error handled in subscribe'

D undefined

💡 Hint

Look at when errorMessage is set inside catchError.

The errorMessage is set inside the catchError operator before rethrowing the error. So after the HTTP error, errorMessage will contain 'Server error occurred'.

🧠 Conceptual

expert

Which statement about Angular HTTP error handling is true?

Select the correct statement about handling HTTP errors in Angular using HttpClient and RxJS.

A Angular requires try-catch blocks around HTTP calls to catch errors.

B Errors must be handled only inside subscribe's error callback for proper catching.

C HttpClient automatically retries failed requests without extra code.

D Using catchError inside pipe allows transforming or recovering from errors before subscription.

💡 Hint

Think about where RxJS operators like catchError work in the observable chain.

Option D is correct because catchError lets you handle errors before the observable reaches the subscriber, allowing recovery or transformation. Option D is wrong because errors can be handled earlier with catchError. Option D is false; retries require explicit operators. Option D is false; try-catch does not work with asynchronous observables.