Unit testing
Unit testing checks if an isolated part of the application functions correctly: does the piece of code or component generate the output you expected?
Writing unit tests helps front-end developers to improve the quality of the code and can prevent future changes from breaking functionality. Every time a new piece of code is written a new unit test can be run to check if everything still works.
Where unit testing tests an isolated piece of the application, end-to-end testing tests the flow of a whole product.