Further reading

Here is a list of information you can refer to:

1. Why testing is important, Atlassian: https://www.atlassian.com/blog/software-teams/why-software-testing-is-important
2. Why is software testing necessary?, TryQA: http://tryqa.com/why-is-testing-necessary/
3. Types of Software Testing: Different Testing Types with Details, Software Testing Help: https://www.softwaretestinghelp.com/types-of-software-testing/
4. Software testing, functional testing: http://softwaretestingfundamentals.com/functional-testing/
5. Test early and Often, Microsoft: https://docs.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2012/ee330950(v=vs.110)
6. Unit Testing, Martin Fowler: https://www.martinfowler.com/bliki/UnitTest.html
7. BDD, Wikipedia: https://en.wikipedia.org/wiki/Behavior-driven_development
8. RSpec: http://rspec.info/