I can run the original function and capture its output. Then I can write an updated function, see what its output is, I can compare them visually to ensure the change I made works as intended.

When you don't have a REPL the only way to do this is by writing a test and running it once you've made the change.

In fact, you can use the code from the tests you write in the REPL to create a unit test for later. The difference is that the REPL feedback loop is much tighter.