Some useful resources and examples on testing.

Test Setup with SSR

NextJS deploys an application in 2 parts:

• Server Side: Runs in nodeJS
• Client Side: Runs in the browser

Proper config requires that test environments are set accordingly. Recall that Node and the Browser are NOT functionally identical. Node does not perfectly replicate all items in the DOM and the browser does not have direct access to things like the DB.

As of December 1, 2021, we configure .tsx files to run in the browser, and .ts to run in node. You can configure this behaviour in the testMatch param in the jest.config.js file

Rendering and Searching for Elements in the DOM

Use the render method from testing library around a React component. Here's a good example:

import React from 'react';
import axios from 'axios';
import { MenuLinks } from '../../../../../src/components/global/Header/Menu';
import { useSession } from 'next-auth/react';
import { BANKLESS } from '../../../../../src/constants/Bankless';
import { guilds as guildsStub } from '../../../../stubs/guilds.stub';
import { render, screen } from '@testing-library/react';

jest.mock('next-auth/react', () => ({
	useSession: jest.fn(),
}));

it('Shows the selector if signed in and if there are customers', () => {
		
		// stub API calls using jest.spyOn
		jest.spyOn(axios, 'get').mockReturnValue(Promise.resolve({
			data: guildsStub
			}
		));

		// using the mockImplementation functionality to replace useSession
    const useSessionTest = useSession as typeof useSession & any;
		useSessionTest.mockImplementation(() => ({ status: 'Done', data: {} }));

		// using mockReturnValueOnce when we have multiple useState calls
		jest.spyOn(React, 'useState')
			.mockReturnValueOnce([[BANKLESS], () => console.log('setCustomers')])
			.mockReturnValueOnce([guildsStub, () => console.log('setGuilds')]);
    
		// call the render method on the element to start the test properly	
		render(<MenuLinks isOpen={false}/>);
	
		// <https://testing-library.com/docs/queries/about>
		// 'role' is predefined by element, you can set the name by using
		// aria-label
		const input = screen.getByRole('combobox', { name: 'dao-selector' });
		expect(input).toBeVisible();

	});

Appendix: Some general notes on Jest

This is for Vue JS but might be of help

Jest and Vue: Things to watch for