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Been there, done that, would not recommend
(feddit.org)
Its definitely a bad idea writing new code that builds up on your old code, that has not been tested properly, because you quickly have to start debugging multiple layer is code at once.
Writing the tests first also ensures that the test actually fails when you expect it to. I've seen test suites that were silently failing for years because they were (presumably) written after the fact and people just assumed that they tested what they said they did. Went in for some other clean up, stared at the test for 10 minutes wondering "how did this ever pass", and then came to realize that test assertions in Jest inside a forEach apparently don't run in the context of the test and failures won't make the test fail. Changing the forEach to a for...of made it all fail immediately.