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Versão: 29.7

Manipulação DOM

Outra classe de funções que é frequentemente considerada difícil de testar é o código que manipula diretamente o DOM. Vamos ver como podemos testar o seguinte trecho de código jQuery, que escuta um evento de clique, busca alguns dados de forma assíncrona e define o conteúdo de um span.

displayUser.js
'use strict';

const $ = require('jquery');
const fetchCurrentUser = require('./fetchCurrentUser.js');

$('#button').click(() => {
  fetchCurrentUser(user => {
    const loggedText = 'Logged ' + (user.loggedIn ? 'In' : 'Out');
    $('#username').text(user.fullName + ' - ' + loggedText);
  });
});

Again, we create a test file in the __tests__/ folder:

__tests__/displayUser-test.js
'use strict';

jest.mock('../fetchCurrentUser');

test('displays a user after a click', () => {
  // Set up our document body
  document.body.innerHTML =
    '<div>' +
    '  <span id="username" />' +
    '  <button id="button" />' +
    '</div>';

  // This module has a side-effect
  require('../displayUser');

  const $ = require('jquery');
  const fetchCurrentUser = require('../fetchCurrentUser');

  // Tell the fetchCurrentUser mock function to automatically invoke
  // its callback with some data
  fetchCurrentUser.mockImplementation(cb => {
    cb({
      fullName: 'Johnny Cash',
      loggedIn: true,
    });
  });

  // Use jquery to emulate a click on our button
  $('#button').click();

  // Assert that the fetchCurrentUser function was called, and that the
  // #username span's inner text was updated as we'd expect it to.
  expect(fetchCurrentUser).toHaveBeenCalled();
  expect($('#username').text()).toBe('Johnny Cash - Logged In');
});

We are mocking fetchCurrentUser.js so that our test doesn't make a real network request but instead resolves to mock data locally. Isso garante que nosso teste pode ser concluído em milissegundos, em vez de segundos e garante uma rápida iteração de teste unitário.

Also, the function being tested adds an event listener on the #button DOM element, so we need to set up our DOM correctly for the test. jsdom and the jest-environment-jsdom package simulate a DOM environment as if you were in the browser. This means that every DOM API that we call can be observed in the same way it would be observed in a browser!

To get started with the JSDOM test environment, the jest-environment-jsdom package must be installed if it's not already:

npm install --save-dev jest-environment-jsdom

The code for this example is available at examples/jquery.