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Angular CLI example for Jest Preview - the visual debugger for Jest

Recently, I contributed an Angular CLI example for Jest Preview in collaboration with its creator, Hung Viet Nguyen.

Jest Preview

Jest Preview is a visual debugger for Jest component tests. It runs a preview server and either displays the rendered DOM of a failed test when the autoPreview option is set or the test we explicitly mark for debugging using the debug function, for example:

// counter.component.spec.ts
import { render, screen } from '@testing-library/angular';
import userEvent from '@testing-library/user-event';
import preview from 'jest-preview';
import { CounterComponent } from './counter.component';
import './counter.component.css'; 

describe(CounterComponent.name, () => {
  it('initially has a zero count', async () => {
    const user = userEvent.setup();
    await render(CounterComponent);

    // Open http://localhost:3336 to see preview
    // Require to run `jest-preview` server before
    preview.debug(); // 👈

    expect(await screen.findByTestId('count')).toContainHTML('0');
  });
});
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Jest Preview displaying the counter component with a count of 0

Angular CLI integration

The Angular CLI guide for Jest Preview refers to the common installation and usage instructions while describing Angular-specific configuration and usage instructions.

While there is room for improvement, we documented workarounds in the guide. Jest Preview is not currently able to automatically transform styleUrls in component tests, support inline component styles, or automatically import application-wide stylesheets. Because of this, we have to add stylesheet import statements in our tests to match the external stylesheet used by the component-under-test.

For example, say we have this counter component:

// counter.component.ts
import { Component } from '@angular/core';

@Component({
  selector: 'app-counter',
  standalone: true,
  styleUrls: ['./counter.component.css'], // 👈
  template: `
    <button type="button" (click)="onCount()">
      The count is:
      <div data-testid="count">{{ count }}</div>
    </button>
  `,
})
export class CounterComponent {
  count = 0;

  onCount(): void {
    this.count += 1;
  }
}
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We would have to add an import statement for ./counter.component.css in our component test:

// counter.component.spec.ts
import { render, screen } from '@testing-library/angular';
import userEvent from '@testing-library/user-event';
import preview from 'jest-preview';
import { CounterComponent } from './counter.component';
import './counter.component.css'; // 👈

describe(CounterComponent.name, () => {
  it('displays multiple counts', async () => {
    const user = userEvent.setup();
    await render(CounterComponent);

    await user.click(await screen.findByRole('button'));
    await user.click(await screen.findByRole('button'));

    // Open http://localhost:3336 to see preview
    // Require to run `jest-preview` server before
    preview.debug();

    expect(await screen.findByTestId('count')).toContainHTML('2');
  });
});
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Jest Preview displaying the counter component with a count of 2

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