Testing HTTP calls with HttpTestingController in Angular

Introduction

We use HttpTestingController to check if our Angular app makes the right HTTP requests without actually calling a server.

You want to test if your service sends the correct GET or POST request.

You want to simulate server responses to see how your app reacts.

You want to make sure no unexpected HTTP calls happen during a test.

Syntax

Angular

import { HttpTestingController, HttpClientTestingModule } from '@angular/common/http/testing';

beforeEach(() => {
  TestBed.configureTestingModule({
    imports: [HttpClientTestingModule],
    providers: [YourService]
  });

  httpTestingController = TestBed.inject(HttpTestingController);
  service = TestBed.inject(YourService);
});

// In your test
const req = httpTestingController.expectOne('url');
expect(req.request.method).toBe('GET');
req.flush(mockData);

httpTestingController.verify();

Always import HttpClientTestingModule to use HttpTestingController.

Call httpTestingController.verify() to check no unexpected requests remain.

Examples

This checks a GET request to 'api/data' and sends back mock data.

Angular

const req = httpTestingController.expectOne('api/data');
expect(req.request.method).toBe('GET');
req.flush({id: 1, name: 'Test'});

This checks a POST request to 'api/save' and returns a success response.

Angular

const req = httpTestingController.expectOne('api/save');
expect(req.request.method).toBe('POST');
req.flush({success: true});

Sample Program

This test checks that DataService.getData() sends a GET request to 'api/data' and receives the expected mock data.

Angular

import { TestBed } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';

@Injectable()
class DataService {
  constructor(private http: HttpClient) {}

  getData() {
    return this.http.get<{id: number; name: string}>('api/data');
  }
}

describe('DataService with HttpTestingController', () => {
  let service: DataService;
  let httpTestingController: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [DataService]
    });

    service = TestBed.inject(DataService);
    httpTestingController = TestBed.inject(HttpTestingController);
  });

  it('should fetch data with GET request', (done) => {
    const mockResponse = {id: 1, name: 'Test Item'};

    service.getData().subscribe(data => {
      expect(data).toEqual(mockResponse);
      done();
    });

    const req = httpTestingController.expectOne('api/data');
    expect(req.request.method).toBe('GET');
    req.flush(mockResponse);

    httpTestingController.verify();
  });
});

Important Notes

Use expectOne() to find a single HTTP request by URL.

Use flush() to simulate the server sending back data.

Always call verify() to ensure no unexpected HTTP calls remain after the test.

Summary

HttpTestingController helps test HTTP calls without real servers.

Use expectOne() to check requests and flush() to send mock responses.

Always verify no extra requests remain with verify().

Practice

1. What is the main purpose of HttpTestingController in Angular testing?

easy

A. To mock and verify HTTP requests without calling a real server

B. To create real HTTP requests to test backend APIs

C. To replace Angular services with fake implementations

D. To automatically generate HTTP request logs during tests

Solution

1. Step 1: Understand HttpTestingController role

   It is designed to intercept HTTP requests in tests and provide mock responses.

2. Step 2: Differentiate from real HTTP calls

   It does not send real requests but simulates them for testing purposes.

3. Final Answer:

   To mock and verify HTTP requests without calling a real server -> Option A

4. Quick Check:

   HttpTestingController mocks HTTP calls = B [OK]

Hint: HttpTestingController mocks HTTP calls, no real server needed [OK]

Common Mistakes:

• Thinking it sends real HTTP requests
• Confusing it with service mocking
• Assuming it logs requests automatically

2. Which of the following is the correct way to inject HttpTestingController in an Angular test?

easy

A. const httpMock = inject(HttpTestingController, TestBed);

B. const httpMock = new HttpTestingController();

C. const httpMock = HttpClientTestingModule.get(HttpTestingController);

D. const httpMock = TestBed.inject(HttpTestingController);

Solution

1. Step 1: Recall Angular TestBed injection syntax

   Use TestBed.inject() to get service instances in tests.

2. Step 2: Check each option

   Only const httpMock = TestBed.inject(HttpTestingController); uses correct syntax: TestBed.inject(HttpTestingController).

3. Final Answer:

   const httpMock = TestBed.inject(HttpTestingController); -> Option D

4. Quick Check:

   Use TestBed.inject() for services in tests = D [OK]

Hint: Use TestBed.inject() to get HttpTestingController instance [OK]

Common Mistakes:

• Trying to instantiate HttpTestingController with new
• Using incorrect module methods
• Passing wrong parameters to inject

3. Given this test snippet, what will req.request.method output?

const req = httpMock.expectOne('/api/data');
console.log(req.request.method);

medium

A. 'GET' if the tested service made a GET request to '/api/data'

B. 'POST' regardless of the actual request method

C. Throws an error because request is undefined

D. 'PUT' if the tested service made a PUT request to '/api/data'

Solution

1. Step 1: Understand expectOne returns a TestRequest

   TestRequest has a request property with HTTP method info.

2. Step 2: The method reflects the actual HTTP call

   If the tested service called GET on '/api/data', req.request.method is 'GET'.

3. Final Answer:

   'GET' if the tested service made a GET request to '/api/data' -> Option A

4. Quick Check:

   req.request.method matches actual HTTP method = A [OK]

Hint: expectOne().request.method shows actual HTTP method used [OK]

Common Mistakes:

• Assuming method is always POST or PUT
• Thinking request property is undefined
• Confusing expectOne with expectNone

4. What is the likely cause of this error in an Angular HTTP test?

Error: Expected one matching request for criteria "Match URL: '/api/items'", found none.

medium

A. The test forgot to call httpMock.verify()

B. The tested service did not make any HTTP request to '/api/items'

C. HttpTestingController was not injected properly

D. The URL in expectOne has a typo but the request was made correctly

Solution

1. Step 1: Analyze the error message

   It says no matching request was found for '/api/items'.

2. Step 2: Understand expectOne behavior

   expectOne throws if no request matches the URL, meaning no request was made.

3. Final Answer:

   The tested service did not make any HTTP request to '/api/items' -> Option B

4. Quick Check:

   No matching request means no HTTP call made = C [OK]

Hint: No matching request means tested code didn't call that URL [OK]

Common Mistakes:

• Assuming verify() missing causes this error
• Thinking injection failure causes this error
• Ignoring URL typos in expectOne

5. In a test, you want to verify that exactly one GET request to '/api/users' was made and respond with mock data. Which code snippet correctly does this using HttpTestingController?

hard

A. const req = httpMock.expectOne('/api/users');
req.error(new ErrorEvent('Network error'));
httpMock.verify();

B. httpMock.expectNone('/api/users');
httpMock.verify();

C. const req = httpMock.expectOne({method: 'GET', url: '/api/users'});
req.flush([{ id: 1, name: 'Alice' }]);
httpMock.verify();

D. const req = httpMock.expectOne('/api/users');
req.flush('');

Solution

1. Step 1: Use expectOne to find the GET request

   expectOne({method: 'GET', url: '/api/users'}) finds the single matching request.

2. Step 2: Respond with mock data using flush

   Calling req.flush with mock user array simulates a successful response.

3. Step 3: Call verify to ensure no unexpected requests

   httpMock.verify() confirms all requests were handled.

4. Final Answer:

   const req = httpMock.expectOne({method: 'GET', url: '/api/users'}); req.flush([{ id: 1, name: 'Alice' }]); httpMock.verify(); -> Option C

5. Quick Check:

   expectOne + flush + verify = A [OK]

Hint: Use expectOne, flush mock data, then verify no extra requests [OK]

Common Mistakes:

• Using expectNone instead of expectOne
• Calling error instead of flush for success
• Not calling verify after flush