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Behavior Driven Development

2:47 with Guil Hernandez and Joseph Fraley

Behavior Driven Development (BDD) is an approach to building software. In BDD, instead of writing code and then seeing if it works, you write tests first and code second. This means you can use tests as an outline for the application code you actually want to write.

Video review

  • Behavior Driven Development is like creating a plan before you write your program
  • Start by describing how the program should work, then write code until it works the way you expect it to work
  • If your tests are good, you can be confident that your functions really do what they’re supposed to do

Red > Green > Refactor

  1. Write the tests, even though at first they’ll fail
  2. Fix the function in the first way that comes to mind, just to get your tests passing
  3. Go back and refactor the function for improvements, repeating the cycle until you’re ready to move on

So we're almost ready to dig into writing some unit tests. 0:00
I hope you're excited as I am. 0:03
Before we do I want to talk about Behavior Driven Development or BDD. 0:05
BDD isn't a type of testing like unit testing or 0:10
integration testing, instead BDD is an approach to building software. 0:13
If you've ever written an essay or 0:18
a story you probably noticed that it's really hard when you don't have a plan. 0:19
You forget what you wanted to say or 0:24
you don't say quite what you mean even if you spend a lot of time writing. 0:26
So it helps to start your writing process with a brainstorm or an outline, so 0:31
that you can always have an idea where you're headed. 0:35
Behavior driven development is like creating a plan 0:38
before you write your program. 0:41
Instead of writing a bunch of code and then seeing if it works, 0:43
like a lot of programmers do, with BDD you write test first and code second. 0:46
Now this may seem counter-intuitive but 0:52
BDD makes a start by describing the behavior of parts of a program. 0:54
How should this function behave when something happens? 0:59
What should we expect to get back from this function if we give it a number 1:02
instead of a string? 1:05
In other words we start by describing how the program should work, 1:07
these are our tests. 1:11
And then we write code until it does work the way we expect it to. 1:12
[SOUND] At first our expectations will not be met, 1:16
because the functions we're testing don't even exist yet. 1:19
So the output of these errors will be bright red, and that's okay. 1:23
Then step by step we code our function until the tests pass, 1:27
that is until bright red errors are replaced with passing green check marks. 1:32
As we fix the errors one by one we get new errors that give us 1:37
guidance in writing the next piece of our function. 1:41
At first you don't worry about the code looking good or 1:44
being efficient you just want to see the test pass. 1:47
If our tests are good we can be confident that our functions 1:51
really do what they're supposed to. 1:54
At this point we can now go back to our function and 1:55
change things around to make it nicer to read or 1:58
faster to run, this process of improving our code is called refactoring. 2:01
Now, the great part of this process is that once we have our tests in place, 2:06
we'll immediately know when a change to our code breaks our 2:11
application's functionality, a red error indicating a failing test. 2:13
This development cycle is called Red Green Refactor. 2:18
First, write the tests, even though at first they'll fail, they might not 2:22
even compile correctly at first if they refer to things that don't exist yet. 2:26
Second, base the function in the first way that comes to mind 2:31
just to get our test, passing. 2:34
And third, go back and re-factor the function for 2:36
improvements, repeating the cycle until we're ready to move on. 2:39
In the next video, I'll show you exactly how to do this with some clear examples. 2:43

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