It's a tool like any other and I reach for it when tests will help me write code faster and at a higher level of quality. Which is pretty often with new code.

It's a tool like any other and I reach for it when tests will help me write code faster and at a higher level of quality. Which is pretty often with new code. Also always before a refactor. Document all the existing states and input and output and I can refactor ruthlessly, seeing as soon as I break something. Tests are also great documentation for how I intend my api to be used. A bunch of examples with input, output, and all the possible exceptions. The first thing I look for when trying to understand a code base are the tests. When do I not write tests? When I'm in the flow and want to continue cranking out code, especially code that is rapidly changing because as I write I'm re-thinking the solution. Tests will come shortly after I am happy with a first prototype in this case. And they will often inform me what I got wrong in terms of how I would like my api consumed. When did it fail me? There are cases when it's really difficult to write tests. For example, Jest uses jsdom, which as an emulator has limitations. Sometimes it is worth it to work around these limitations, sometimes not. Sometimes a dependency is very difficult to mock. And so it's not worth the effort to write the test. Tests add value, but like anything that adds value, there is a cost and you have to sometimes take a step back and decide how much value you'll get and when the costs have exceeded the value and it's time to abandon that tool.