Depends on what you mean be effective. TDD is great if you want to be sure you don’t break things in the future. Repl seems better for getting the next thing working right now.
The repl is great for understanding something quickly and easily. It works for big new parts of the code base and small things that you just need to be reminded about. The nice thing is, if you save your repl output, you can really easily create tests from it.