TDD. It stands for Test Driven Development. Personally, I don’t care for it1.

Yet when I job-hunt, I use it religiously for interviews where I code2.

Wait, why?

Any technical challenge is actually two challenges.

  1. Can you take a broadly-worded problem and nail it down into something unambiguous?
  2. Can you implement challenging but unambiguous spec in the allotted time?

TDD forces you into the ideal mindset for nailing down (1) problem definitions. There’s no better way to properly grok a problem than to have to think through all the fun ways an implementation could be slightly off.

The implementation itself (2) also gets easier since you no longer have to assume that your code “probably” works, or fiddle with a REPL each time you’re ready to check.

So how do I do it?

As soon as you hear the problem, resist the urge to rush straight to the solution.

Instead, force yourself to fully think through the test cases for an arbitrary implementation. Make it a dialog - “what if the same element is in the array multiple times?” Get the interviewer engaged. Show you care.

By the time you’ve written good tests and the interviewer agrees, you’re all set. Even though you tried not to, you probably already have a decent solution in mind. Your tests will let you know you’re done. They’ll also give you comfort that subsequent clean up / refactor didn’t inadvertantly break your implemenation.

Yeah, but I can always just write tests later.

When I conduct interviews, I keep running into candidates clever enough to power through engineering challenges on raw talent3. The candidate implements an optimal solution within fifteen minutes. Then it’s time for tests and she just cannot think of cases beyond the common path, which makes her look like a cowboy.

I blame the curse of knowledge - once you’ve solved the problem, it’s harder to think about what wrong turns you could have taken. It all feels trivial.

Seriously though, are you going to make me import a testing framework? Like, Mocha or Cucumber? Ugh.

¯\_(ツ)_/¯, I just use asserts and print statements. If I’m feeling fancy, I’ll create an array of test case scenarios and run them like

test_cases = [
  [[2, 2], 4],
  [[1, 2, 3, 4], 10],
  [[], 0],
  [[-3, 2], 1],
  [[100, 2], 102],
  [["banana"], None]
]

for test_case in test_cases:
  args, expected_answer = test_case
  answer = quantum_addition(args)

  print("called add({}) and got {}, expected {}"
    .format(args, answer, expected_answer))

  if answer != expected_answer:
    print("UH OH!")

  # (or you could just do: assert answer == expected_answer)

If you insist on being fancy, you can do that thing where you ask your interviewer if they’d like you to use a proper testing framework first, and of course they don’t, but now it looks like you could and you’re just doing them a favor by using asserts, cause you’re like, super assertive.

Ok so now what?

Well obviously you should come work at Opendoor with me! We have the good coconut water and don’t run out of it until like noon, 12:30 on good days. Plus, there’s a decent chance you’ll run into me on your phone screen and we can that awkward interaction of trying to figurue out why my name sounds familiar.


Thanks to Vaibhav, Joe, Nick, Gayle, Tess, Charlie, Kevin, Zain, and Yahel for giving feedback on earlier drafts that I incorporated.

  1. Don’t get me wrong, TDD has its uses, but we often mistake what Kent Beck meant by ‘unit’. Testing ‘just right’ is hard.

  2. To be clear, TDD won’t help you with interview where you don’t actually code (IE, whiteboard interviews) You’ll still have to start by asking questions, though.

  3. I used to have that problem earlier in my career. I’ve addressed it, largely by becoming dumber.