https://martinfowler.com/bliki/TestDrivenDevelopment.html
Quoting:
"Test-Driven Development (TDD) is a technique for building software that guides software development by writing tests. It was developed by Kent Beck in the late 1990's as part of Extreme Programming. In essence you follow three simple steps repeatedly:
Write a test for the next bit of functionality you want to add.Write the functional code until the test passes.Refactor both new and old code to make it well structured.
You continue cycling through these three steps, one test at a time, building up the functionality of the system. "
The book (by Kent Beck):
https://www.amazon.com/gp/product/0321146530?ie=UTF8
