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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.
I tried TDD in a personal project recently and it got annoying pretty fast. It was also a project where I tried a new Framework so writing test when one doesn't know how things behave exactly results in adjusting the tests afterwards anyway.
Thinkimg how I want to designe my API upfront while discovering the details as I go served me well in the past (still in context of personal projects).
It also doesn't help when my tests have more bugs than the tested code...