how_to_test.md

🦉 How to test Components 🦉

Content

• Overview
• Unit Tests

Overview

It is a good practice to test applications and components to ensure that they behave as expected. There are many ways to test a user interface: manual testing, integration testing, unit testing, ...

In this section, we will discuss how to write unit tests for components.

Unit Tests

Writing unit tests for Owl components really depends on the testing framework used in a project. But usually, it involves the following steps:

• create a test file: for example SomeComponent.test.js,
• in that file, import the code for SomeComponent,
• add a test case:
  • create a real DOM element to use as test fixture,
  • create a test environment
  • create an instance of SomeComponent, mount it to the fixture
  • interact with the component and assert some properties.

To help with this, it is useful to have a helper.js file that contains some common utility functions:

let lastFixture = null;

export function makeTestFixture() {
  let fixture = document.createElement("div");
  document.body.appendChild(fixture);
  if (lastFixture) {
    lastFixture.remove();
  }
  lastFixture = fixture;
  return fixture;
}

export async function nextTick() {
  await new Promise((resolve) => setTimeout(resolve));
  await new Promise((resolve) => requestAnimationFrame(resolve));
}

With such a file, a typical test suite for Jest will look like this:

// in SomeComponent.test.js
import { SomeComponent } from "../../src/ui/SomeComponent";
import { nextTick, makeTestFixture } from '../helpers';


//------------------------------------------------------------------------------
// Setup
//------------------------------------------------------------------------------
let fixture: HTMLElement;
let env: Env;

beforeEach(() => {
  fixture = makeTestFixture();
});

afterEach(() => {
  fixture.remove();
});

//------------------------------------------------------------------------------
// Tests
//------------------------------------------------------------------------------
describe("SomeComponent", () => {
  test("component behaves as expected", async () => {
    const props = {...}; // depends on the component
    const comp = await mount(SomeComponent, fixture, { props });

    // do some assertions
    expect(...).toBe(...);

    fixture.querySelector('button').click();
    await nextTick();

    // some other assertions
    expect(...).toBe(...);
  });
});

Note that Owl does wait for the next animation frame to actually update the DOM. This is why it is necessary to wait with the nextTick (or other methods) to make sure that the DOM is up-to-date.