Updated: July 14, 2020
Testing
How do you know if the system you built works? Through testing it! But testing different aspects of your system manually (i.e. by entering text into input fields and clicking buttons yourself) can be tedious, and take up a lot of time. This is why developers rely on tools which run through a defined list of expectations to fulfill and alert the developer if one (or many) outcomes did not turn out as expected.
Unit Testing
Unit testing is where individual parts of an application (each 'unit') are tested in isolation from one another. So, for example, in a standard calculator app, you would test the add function first to see if numbers add together and produce an expected result, and then the subtract function to see if numbers subtract from each other and produce an expected result, and then the multiply function, etc.
Tools
Framework
Testing Framework - Developed and maintained by Facebook and contributors. Pretty much the go to for React and can be used alongside React Testing Library.
Tutorials
How to use React Testing Library Tutorial
Read/Tutorial
A great introduction on how to use Kent C Dodd's (who I am a big fan of) testing framework. Everyone should learn at least one thing from this article.
End to End testing
End to end testing is where the relationship between multiple parts of an application are tested. It simulates how the user would use the application, wherein multiple components are involved and rely on one another. As a simple example, when the user clicks on the "hamburger button" (component one), does the correct menu (component two) appear? For a more complex example, when the user submits data through a form (component one), is that data submitted to the database (component two) correctly?
Tools
Easily Write Tests
Cypress io is an open source end to end testing framework for testing anything that runs in the browser. Follow the whole user journey and easily pick up on any issues a long the way.