Automated testing, both of the UI and the back-end service APIs, will make you more confident that everything works as intended and will reduce stress when developing, refactoring, adding new features, or changing existing ones.
Testing back-ends is easy. You take your language of choice, pair it with your favorite framework, write some tests, and hit “run.” Your console says “Yay! It works!” Your continuous integration service runs your tests on every push, life is great. Sure, test-driven development (TDD) is weird at first, but a predictable environment, multiple test runners, test tooling baked into frameworks, and continuous integration support, make life easy. Five years ago I thought tests were the solution to every problem I’ve ever had. Then Backbone got big. We all switched to front-end MVC. Our testable backends became glorified database servers. Our most complicated code moved into the browser. And our apps were no longer testable in practice. That’s because testing front-end code and UI components is kinda hard.