test-driven-development

1# Test-Driven Development (TDD)
2 
3## Overview
4 
5Write the test first. Watch it fail. Write minimal code to pass.
6 
7**Core principle:** If you didn't watch the test fail, you don't know if it tests the right thing.
8 
9**Violating the letter of the rules is violating the spirit of the rules.**
10 
11## When to Use
12 
13**Always:**
14- New features
15- Bug fixes
16- Refactoring
17- Behavior changes
18 
19**Exceptions (ask your human partner):**
20- Throwaway prototypes
21- Generated code
22- Configuration files
23 
24Thinking "skip TDD just this once"? Stop. That's rationalization.
25 
26## The Iron Law
27 
28```
29NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST
30```
31 
32Write code before the test? Delete it. Start over.
33 
34**No exceptions:**
35- Don't keep it as "reference"
36- Don't "adapt" it while writing tests
37- Don't look at it
38- Delete means delete
39 
40Implement fresh from tests. Period.
41 
42## Red-Green-Refactor
43 
44```dot
45digraph tdd_cycle {
46    rankdir=LR;
47    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
48    verify_red [label="Verify fails\ncorrectly", shape=diamond];
49    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
50    verify_green [label="Verify passes\nAll green", shape=diamond];
51    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
52    next [label="Next", shape=ellipse];
53 
54    red -> verify_red;
55    verify_red -> green [label="yes"];
56    verify_red -> red [label="wrong\nfailure"];
57    green -> verify_green;
58…