Cracking the Code: Angular Test – How to Mock an Imported Const

Are you tired of Angular testing woes? Do you find yourself stuck on how to mock an imported const in your Angular tests? Fear not, dear developer, for we’re about to dive into the world of Angular testing and conquer this challenge together!

Why Mock an Imported Const?

In Angular, const values are often imported from external modules or files. These values can be environment variables, API endpoints, or even hard-coded values. When writing tests, it’s crucial to isolate the component or service being tested from external dependencies. By mocking an imported const, we can control the behavior of our tests and ensure they’re not affected by external factors.

The Problem: Imported Const Interference

Let’s say we have a component that imports a const value from an external file:

import { API_URL } from './api.constants';

In our component, we use the API_URL to make an HTTP request:

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';

@Component({
  selector: 'app-example',
  template: '
Example Component
'
})
export class ExampleComponent implements OnInit {
  constructor(private http: HttpClient) { }

  ngOnInit(): void {
    this.http.get(API_URL).subscribe((response) => {
      console.log(response);
    });
  }
}

When we write a test for this component, we might want to control the response from the API_URL. However, since the API_URL is an imported const, we can’t simply change its value in our test. This is where mocking comes in!

Mocking an Imported Const with Jasmine

Jasmine, the default testing framework in Angular, provides a way to mock objects and values. We can use the ` jasmine.createSpyObj` method to create a mock object, but for imported consts, we need to take a different approach.

Step 1: Create a Mock Const Module

Create a new file called `api.constants.mock.ts` with the following content:

export const API_URL = 'https://example.com/mock-api';

Note that we’re exporting a new value for API_URL. This will be our mock value.

Step 2: Configure the TestBed

In our test file, we need to configure the TestBed to use our mock module instead of the original one:

import { TestBed } from '@angular/core/testing';
import { ExampleComponent } from './example.component';
import { apiConstantsMock } from './api.constants.mock';

describe('ExampleComponent', () => {
  beforeEach(async () => {
    await TestBed.configureTestingModule({
      declarations: [ExampleComponent],
      providers: [
        { provide: './api.constants', useValue: apiConstantsMock }
      ]
    })
    .compileComponents();
  });
});

We’re telling the TestBed to use our mock module instead of the original `api.constants` module.

Step 3: Verify the Mocked Value

Now that our mock module is in place, we can write a test to verify that the API_URL value is being used:

it('should use the mocked API_URL', () => {
  const component = TestBed.createComponent(ExampleComponent);
  const fixture = TestBed.createComponent(ExampleComponent);
  fixture.detectChanges();

  const httpMock = TestBed.inject(HttpClient);
  spyOn(httpMock, 'get').and.returnValue(of({ mockResponse: true }));

  expect(httpMock.get).toHaveBeenCalledTimes(1);
  expect(httpMock.get).toHaveBeenCalledWith('https://example.com/mock-api');
});

We’re verifying that the `http.get` method is called with our mocked API_URL value.

import { overseerrable } from '@angular/core/testing';

beforeEach(async () => {
  await TestBed.configureTestingModule({
    declarations: [ExampleComponent]
  })
  .override(overseerrable('./api.constants', 'API_URL'), { API_URL: 'https://example.com/mock-api' })
  .compileComponents();
});

it('should use the mocked API_URL', () => {
  const component = TestBed.createComponent(ExampleComponent);
  const fixture = TestBed.createComponent(ExampleComponent);
  fixture.detectChanges();

  const httpMock = TestBed.inject(HttpClient);
  spyOn(httpMock, 'get').and.returnValue(of({ mockResponse: true }));

  expect(httpMock.get).toHaveBeenCalledTimes(1);
  expect(httpMock.get).toHaveBeenCalledWith('https://example.com/mock-api');
});