Test-Driven Development

// add.test.ts
import { add } from "./add";

test("add returns the sum of two numbers", () => {
  expect(add(2, 3)).toBe(5);
});
// ❌ Fails: cannot find module './add'

// add.ts
export function add(a: number, b: number): number {
  return a + b;
}
// ✅ Test passes

// add.ts — no logic change needed here, but you might rename,
// extract constants, or improve types while the suite stays green.
export const add = (a: number, b: number): number => a + b;
// ✅ Still green

# test/math_test.exs  (RED)
test "add/2 returns the sum" do
  assert Math.add(2, 3) == 5
end

# lib/math.ex  (GREEN)
defmodule Math do
  def add(a, b), do: a + b
end

# Run full suite
bun test

# Watch mode while iterating
bun test --watch

# Run a specific file
bun test path/to/file.test.ts

# Elixir variant
mix test

// BAD: production code written first, test added after
export function greet(name: string) { return `Hello, ${name}!`; }
// ... then test written to match existing impl

// GOOD: test written first, impl follows
test("greet returns a personalised greeting", () => {
  expect(greet("Ada")).toBe("Hello, Ada!");
});
// ❌ red → write greet() → ✅ green

// BAD: asserting internal call sequence
expect(mockFormatter.format).toHaveBeenCalledBefore(mockLogger.log);

// GOOD: assert observable output
expect(result).toBe("Hello, Ada!");

// BAD: one test covers create/login/update/delete flow
test("user lifecycle", () => { /* 40 lines */ });

// GOOD: one behavior per test
test("createUser returns a user with the given name", () => { ... });
test("login returns a session token for valid credentials", () => { ... });

// BAD
await sleep(1000);
expect(result).toBeDefined();

// GOOD: synchronize with deterministic signals
await expect(promise).resolves.toBeDefined();

# Evaluate skill quality
sh skills/agentic-harness/skill-quality-auditor/scripts/evaluate.sh test-driven-development --json

# Lint this skill docs
bunx markdownlint-cli2 "skills/test-driven-development/**/*.md"