To complete this tutorial, you will need to have the following installed:

  • Ruby
  • Git
  • A text editor
  • A terminal or command prompt (we will be working mainly from the terminal, if you are not comfortable using yours, you may want to complete a Command Line Crash Course before you continue).

Think you are missing something? Check/Install here.


Using TDD, write a program which will help you cheat at the drinking game fizzbuzz.


  • When the given number is divisible by 3, say fizz
  • When the given number is divisible by 5, say buzz
  • When the given number is divisible by 3 and 5, say fizzbuzz!
  • When the given number does not fit any of the other rules, print the number
$ ./fizzbuzz.rb 3

$ ./fizzbuzz.rb 5

$ ./fizzbuzz.rb 15

$ ./fizzbuzz.rb 7
7 :(

Part 1: Project setup


  1. Open Terminal (or iTerm or whatever else you like to get a command prompt) and create a new directory. Then change into that directory and initialise a new git repository (New to Git? See this guide.):

     cd ~
     mkdir -p workspace/fizzbuzz
     cd workspace/fizzbuzz
     git init
     git remote add origin <URL OF YOUR REPO ONLINE>
     echo "tdd exercise in ruby" >
     git add
     git commit -m ""
     git push -u origin main
  2. Create 2 subdirectories, lib and spec, in your project. lib you will recognise as the normal location of your Ruby code. spec is where we are going to put our tests.

  3. Install rspec. Rspec is the testing framework we are going to use to write unit tests for our code. Rspec is a gem so we need to install it using the gem command.

     gem install rspec

    A gem is a third-party open-source ruby library which we can download to use inside or with our own code. Anyone can make a gem and there are thousands out there; if you create something and think “hey! this is cool and I find it very useful, maybe other people will too”, then you can share it with the world on RubyGems.

Part 2: Our first test

Test Driven Development follows a simple pattern: red -> green -> refactor. In reality, this works as follows:

  1. Write a test which would pass if the code was implemented correctly.
  2. See it fail.
  3. Write just enough code to make the test pass.
  4. See the test pass.
  5. Look over the code and think of ways to improve what you have. Is there any repetition? Can an algorithm be made more efficient?

Writing code this way has 4 benefits: 1) by only writing what you need to achieve basic tasks, you end up writing less code, all of which is used (no ghost functions which you have little memory of); 2) every single function is tested, which makes adding more or changing bits a breeze as you will find out immediately what you may have broken; 3) nicely structured tests makes it easy for others looking at your project to figure out what your code is supposed to do (a good way to get contributors); and 4) because of 1 and 2, you have complete confidence that your code does what it is supposed to do.

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So let’s get going with our first test:


  1. Create a new file called fizzbuzz_spec.rb in your spec directory.

  2. Open the project in your text editor, and write the following to it:

     describe 'Fizzbuzz' do
         context 'knows when a number' do
             it 'is divisible by 3' do

    So what have we done here? You’ll notice that we haven’t just written it 'works'. TDD is about ensuring that you write your code incrementally by breaking down your problem into small chunks and tackling each one at a time. Right now we are writing a pretty simple game, so you may be thinking TDD is overkill, but when it comes to a large project or a very complex problem which you can’t possibly envisage how it will turn out, TDD is a very useful discipline to learn.

    So let’s read our test. The first line is to indicate what you are testing. The rspec program recognises everything within this describe block as the scope of what you want to test. The bit in the quotes is for your benefit. On the next line we declare a context. Again the quotes are for your benefit. Rspec understands that tests under the same context are grouped together and share the same state. The it block will contain your test (right now there is nothing actually being tested).

  3. Now we have to state what we expect to happen when a function in our code runs. We have not written a function yet, but we have explained what we want in our test, so we know vaguely what it should look like. Put the following line inside your it block:

         it 'is divisible by 3' do
             expect(is_divisible_by_three?(3)).to be true
  4. Run the test: rspec spec/fizzbuzz_spec.rb This should fail with something like undefined method 'is_divisible_by_three?'. Which makes sense; we haven’t defined that method anywhere. Time to write some code.

How your project should look at this stage.

Part 3: Make it green

Now that we have our first failing test we are going to follow the errors given by rspec to make it pass. So let’s start with the first error we were given: undefined method 'is_divisible_by_three?'


  1. Create and open a new file called fizzbuzz.rb in your project lib directory.

  2. Define that function which rspec was complaining about. Don’t make it do anything, just define it:

     def is_divisible_by_three?(number)
  3. Save the file and run the test again. Same thing? What are we missing? Does our test file know where our code file is? We should probably require it at the top of spec/fizzbuzz_spec.rb

     require_relative '../lib/fizzbuzz.rb'
     describe 'Fizzbuzz' do
  4. Run the test again: rspec spec/fizzbuzz_spec.rb. Now we should see something new:

     1) fizzbuzz knows when a number... is divisible by three
        Failure/Error: expect(is_divisible_by_three?(3)).to be true
          expected true
           got nil
        # ./spec/fizzbuzz_spec.rb:8:in `block (3 levels) in <top (required)>'

    Our test is expecting our function to return true but it is getting nil. So let’s go give it what it wants.

  5. Go back to lib/fizzbuzz.rb and make is_divisible_by_three? return true

     def is_divisible_by_three?(number)
  6. Run the test again. And we’re green! Congrats, you have passed your first test. Let’s go wreck it.

How your project should look at this stage.

Part 4: Make it red

Obviously we are not done yet. Hardcoding true like that is seen as a Very Bad Thing, and also not very useful for our game. So let’s write another test to force ourselves to do the right thing.


  1. In spec/fizzbuzz_spec.rb add a second it block under the first. (make sure you stay in the same context block.)

     it 'is NOT divisible by 3' do
         expect(is_divisible_by_three?(1)).to be false
  2. Run the tests again. Back to red? 2 examples 1 failure? Expected false got true? Excellent. Time for some maths.

  3. Back in lib/fizzbuzz.rb we need to make our function work out whether the number it is being passed as an argument (which right now we are ignoring) is actually divisible by three. To do that we need to use modular arithmetic: if number can be evenly divided by 3, it should return 0 (i.e. not have a remainder).

     def is_divisible_by_three?(number)
         number % 3 == 0
  4. Now when we run the tests, both should pass. Seeing as we have very little code right now, there is no refactoring to be done so we can go on to writing more tests.

How your project should look at this stage.

Part 5: Around we go again…


  1. In spec/fizzbuzz_spec.rb, add another it block (again still within the same context block) to test whether numbers are divisible by 5:

     it 'is divisible by 5' do
         expect(is_divisible_by_five?(5)).to be true
  2. Run the tests. Do you see undefined method 'is_divisible_by_five?'?

  3. Go define is_divisible_by_five? in lib/fizzbuzz.rb. (just define, don’t make it do anything.)

  4. Run the tests. expected true, got nil? Make your function return true.

  5. Run the tests. Green again? Go back to your test file and write the opposing it 'is NOT divisible by 5' block.

  6. Run the tests. expected false got true? Fix your code to make it pass.

2 functions in and we are starting to see a pattern here, but let’s leave off refactoring just a little while longer and move onto the last calculation. By now you should know the routine, so go ahead and write 2 more tests for a function which knows if a number is_divisible_by_three_and_five.

Once you have all 6 tests passing, commit your work and push to github:

git add lib/ spec/
git commit -m "knows if numbers are divisible by 3, 5 or 3 and 5"
git push

How your project should look at this stage.

Part 6: First Refactor

Right now we have three functions which are doing more or less the same thing. Let’s see if we can DRY this out a bit.

  1. In lib/fizzbuzz.rb, edit your code so that the 3 is_divisible_by_*? functions are replaced by just 1.

     def is_divisible_by?(number, divisor)
         number % divisor == 0
  2. Run your tests. Are they very unhappy? Update them to use the new function. If you are having trouble making them pass, remember to read the failure messages carefully: rspec is very helpful and will generally point you in the right direction.

How your project should look at this stage.

Part 7: FizzBuzz says

So now our code can tell us whether a number is divisible by 3, 5 or 3 and 5, but we can’t really play the game with this.


  1. In spec/fizzbuzz_spec.rb define a new context block underneath the end of the old one.

     context 'when playing the game, fizzbuzz says...' do
         it '"fizz" when a number is divisible by 3' do
  2. Next, put what you would expect to happen inside your it block:

     it '"fizz" when a number is divisible by 3' do
       expect(fizzbuzz_says(3)).to eq 'fizz'
  3. Run your tests. They should fail in a way which by now should be familiar.

  4. Go into lib/fizzbuzz.rb and create that function. (just define. don’t implement.)

  5. Run the tests again and follow the error message, remember to do just enough to make them pass.

     def fizzbuzz_says(number)
  6. Add the next test to force yourself to write code which actually evaluates something.

     it '"buzz" when a number is divisible by 5' do
       expect(fizzbuzz_says(5)).to eq 'buzz'
  7. See the test fail, then go back to your code and make your new function process the argument it is passed by using our is_divisible_by function:

     def fizzbuzz_says(number)
         return "fizz" if is_divisible_by?(number, 3)
         return "buzz" if is_divisible_by?(number, 5)

    note: we actually have to explicitly return here, otherwise the program will process all lines in the function and only return the result of the last one. Remove the returns and run the tests without them. You should see your ‘fizz’ test failing, since the last line it evaluated returned false when the number 3 was found not to be divisible by 5. Therefore the fizzbuzz_says function overall returns nil.

  8. Go back to spec/fizzbuzz_spec.rb, and add the test which check that the program says “fizzbuzz” when a number is divisible by 3 and 5.

  9. Run the tests, watch it fail.

  10. Go to your code and make it pass.

    note: remember to watch your ordering here. Since you are returning immediately when the number is first sucessfully divisible, you may end up saying “fizz” rather than “fizzbuzz”. Make sure you check if a number can be divisible by 3 and 5 first. Switch the order of your code to see your tests failing in this way.

  11. The last thing we need to do is write a test (and then the code) for when the number is not divisible by 3 or 5. It should just return the number.

Once you have all 10 tests passing, commit your work and push to github:

git add lib/ spec/
git commit -m "says fizz, buzz and fizzbuzz"
git push

How your project should look at this stage.

Part 8: Bonus Round

All our calculations are done and we are ready to play the game… but it doesn’t quite work in the way we planned at the start. We can’t do $ ./fizzbuzz.rb 3 and expect to see fizz in the terminal right now.

So, for bonus points, you are going to write some integration tests. Up to now we have been testing out each individual unit of code in… unit tests (obvs). Now we need to verify that our code integrates with the tools which are not directly part of that code but are going to interact with it (in this case the command line).

Normally, you would write your integration tests in a separate place (the code too), but since we are only going to write three more we can just add them to the very end of our existing test file in a new describe block.

You will need to find a way of executing your game from inside a test file. Things like ShellOut or Rspec Command may help you do that, but there are others so take some time to look around.

Drive out your code the same way as you did above: describe the first simple thing you expect to happen, do just enough to make it pass, then move onto the next simple thing. Remember to follow the errors returned by rspec; 8 times out of 10 the answers will be there.

To make our game work properly, we have to test that:

  1. Our code can be executed (successfully, with exit status 0).
  2. The result will be printed to terminal output (stdout).
  3. We can pass mulitple arguments and see them all processed (./fizzbuzz 1 2 3 should print 1 2 fizz).

Once you have all 13 tests passing, commit your work and push to github:

git add lib/ spec/
git commit -m "can be run from the command line"
git push

Here is one way your project could look after your first, second, and third tests.


And that’s it! You just test-drove your first program.

But don’t stop there; test-driven development is a good habit to get into and the majority of (sensible) companies value it very highly. Think back to small programs you have written and see if you can do them again through TDD. Or test drive out another simple game or calculator (Leap Year, maybe?).

If you are not a complete newbie to coding, go ahead and find out how you would use something like capybara with rspec to test drive your sinatra webapp.

There is a testing framework (often much more than one) for every language, so go ahead and play Fizzbuzz again in the one of your choice.

In fact, small exercises like Fizzbuzz are a great way to get to grips with a new language and its testing framework; it’s my personal goto for the first thing I write in whatever new thing I am trying.

At some point I will be going on to write another, more challenging TDD tutorial so watch this space. :)


Mistakes? Corrections? Suggestions?

Is something unclear? Do you need help?