Overview

The idea is to automate the manual testing of code, with code.

This requires an initial time investment, which is quickly paid back in the time saved by making sure the application is stable.

Test types

Unit tests
Testing single functions with no dependencies (thousands).
Integration test
Testing single functions with dependencies i.e. functions that call other functions (couple) Ex. Testing APIs. Does the combination of single functions work?
End-to-end (E2E) / UI test
Testing by simulating a literal manual UI interaction (few).

The following is needed to test code.

Test runner - Executes tests and summarizes results. Ex. Jest, Mocha.
Assertion library - Define testing logic and conditions. Ex. Jest, Chai.
Headless browser - E2E testing - Simulate browser interactions. Ex. Puppeteer, Cypress.

Jest is both a test runner and an assertion library.