Well the question looks more like a why instead of a how.. answering the how is simple, if you are starting on a new project (as you seem to be), start with uni

Well the question looks more like a why instead of a how.. answering the how is simple, if you are starting on a new project (as you seem to be), start with unit tests for all functions, for anything that talks to DBs/external APIs use mocks. Answering the why, scenario 0 - it forces you to have functions that do one thing and one thing well. Unit testing mostly helps people keep functions small and focused. scenario 1 - assume you are working on a fun app without tests. Everything is fine and the app works great. After some time you decide to add a new feature X, and you realize you need to modify an existing function to achieve that.. you modify that, and the new feature X works fine, but two days later you will realize that an old feature Y is broken. scenario 2 - scenario 1 but with collaborators instead of yourself revisiting the app. scenario 3 - people use unit tests to understand code - while reading/working on other people's code. Unit tests also serve as excellent examples on how to call/use something, which params to pass etc. There are a lot more reasons esp for large projects, and you never know when a small one off project grows into something big and critical so its recommended to always start with unit tests as adding them later is usually 10x more work as it would require refactoring etc.