Typed Service Contracts

Red-Green-Refactor (TDD) Skill: TypeScript Edition

This skill implements a structural framework for AI-assisted programming to ensure every line of code is verifiable, typed, and purposeful.

The Three-Phase Cycle

Phase 1: Red (Establish Failure)

You must prove the feature does not exist and that your test is valid.

1. Write One Test: Create a single test case (e.g., in Vitest or Jest) for the next small piece of behavior. 2. Execute & Fail: Run the test. It must fail. 3. Verify: Ensure the failure is related to the missing logic (e.g., ReferenceError: add is not defined) and not a configuration error.

Phase 2: Green (Minimal Pass)

Make the test pass as quickly and simply as possible.

1. Minimal Implementation: Write the simplest code that satisfies the test. Do not build for the future; focus strictly on the current "Red" test. 2. Run Tests: Execute the suite. All tests must be Green. 3. Evidence: The transition from Red to Green is the "Proof of Work" for the developer.

Phase 3: Refactor (Clean Up)

Improve the code structure while maintaining the "Green" state.

1. Clean Up: Improve naming, remove duplication, and optimize the code written in Phase 2. 2. Safety Net: Rerun the tests after every change. If they turn Red, revert the change immediately.

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Core Operational Rules

1. No "Horizontal Splurging"

You are strictly forbidden from writing a large "splurge" of multiple tests at once. You must follow a strictly incremental loop:

• Write 1 Test -> See it Fail -> Write 1 Fix -> See it Pass.
• Repeat this loop for every sub-feature.

2. Impose Backpressure

Use automated assertions and strong typing (TypeScript) as backpressure to prevent the AI from "guessing" the solution or "playing in the mud" with low-quality code.

3. Verification of Integrity

Never modify an existing test to make a failing implementation pass. If a test must change, it must be because the requirement changed, not because the code is difficult to write.

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Example Workflow (TypeScript + Vitest)

Step 1: Red

// math.test.ts
import { describe, it, expect } from "vitest";
import { add } from "./math";

describe("add", () => {
  it("should sum two numbers", () => {
    expect(add(2, 2)).toBe(4); // Fails: ReferenceError: add is not defined
  });
});

Step 2: Green

// math.ts
export const add = (a: any, b: any) => {
  return 4; // Passes: Minimal code to satisfy the test
};

Step 3: Refactor

// math.ts
/**
 * Sums two numbers with explicit type safety.
 */
export const add = (a: number, b: number): number => {
  return a + b; // Passes: Proper implementation with safety net
};