Advanced Testing Techniques

Testing

Testing Asynchronous Code

Vitest and Jest provide utilities to handle asynchronous code. Let’s consider an example of an asynchronous function:

javascript 
	// fetchData.mjs
	export async function fetchData() {
	  return new Promise((resolve) => {
	    setTimeout(() => {
	      resolve(40);
	    }, 1000);
	  });
	}

To test this function, we need to handle the asynchronous nature properly:

javascript 
	// fetchData.test.mjs
	import { describe, it, expect } from 'vitest';
	import { fetchData } from './fetchData';
	
	describe('fetchData.mjs', () => {
	  // Use the `it` function to define a test case
	  it('should fetch data correctly', async () => {
	    // Use the `await` keyword to wait for the asynchronous function to complete
	    const result = await fetchData();
	    // Use the `expect` function to make assertions about the output
	    expect(result).toBe(40);
	  });
	});

Snapshot Testing

Snapshot testing is a technique to test the output of a function by comparing it to a stored snapshot. This is particularly useful for testing UI components. Although Vitest is primarily used for unit testing, it can also handle snapshot testing:

javascript 
	// component.js
	export function renderComponent() {
	  return '<div>Hello World</div>';
	}
	
	// component.test.js
	import { describe, it, expect } from 'vitest';
	import { renderComponent } from './component';
	
	describe('component.js', () => {
	  it('should render the component correctly', () => {
	    const output = renderComponent();
	    expect(output).toMatchSnapshot();
	  });
	});