Wallaby.js is a no-brainer tool for writing frontend unit tests. I see a lot of people at work struggle with unit testing. For the productivity boost it gives, it's easily worth the cost.

Seconded for Wallaby, as I don't often see it mentioned.

And even if you don't need instant feedback on tests whilst you're writing code, it's great during general development - set up a single test with the result you want and start doing some TDD-style coding, and Wallaby doesn't just show you the test outcome but also shows the values of all variables as hints decorating the lines in the editor itself whilst you type, making it much easier to get to working code.