When doing TDD, why should I do "just enough" to get a test passing?

Asked 2010-08-20T20:02:08.450

Looking at posts like this and others, it seems that the correct way to do TDD is to write a test for a feature, get just that feature to pass, and then add another test and refactor as necessary until it passes, then repeat.

My question is: why is this approach used? I completely understand the write tests first idea, because it helps your design. But why wouldn't I create all tests for a specific function, and then implement that function all at once until all tests pass?

unit-testing tdd

Edit Report

2 Answers

The reason behind the principle is simple. How practical it is to stick to is a separate question.

The reason is that if you are writing more code that what is needed to pass the current test you are writing code that is, by definition, untested. (It's nothing to do with YAGNI.)

If you write the next test to "catch up" with the production code then you've just written a test that you haven't seen fail. The test may be called "TestNextFeature" but it may as well return true for all the evidence you have on it.

TDD is all about making sure that all code - production and tests - is tested and that all those pesky "but I'm sure I wrote it right" bugs don't get into the code.

answered 2010-08-20T20:38:11.877

I would do as you suggest. Write several tests for a specific function, implement the function, and ensure that all of the tests for this function pass. This ensures that you understand the purpose and usage of the function separately from your implementation of it.

answered 2010-08-20T20:09:16.703

When doing TDD, why should I do "just enough" to get a test passing?

2 Answers

Your Answer