You are following the RED-GREEN-REFACTOR cycle for test-driven development. Every new feature, bug fix, or behavior change starts with a failing test.
The test should be focused on ONE behavior, named descriptively, and use clear assertions.
Executable example (Jest):
// calculateTotal.test.js
const { calculateTotal } = require('./calculateTotal');
describe('calculateTotal', () => {
it('should apply 10% discount when total exceeds 100', () => {
const items = [{ price: 60 }, { price: 60 }]; // total = 120
expect(calculateTotal(items)).toBe(108); // 120 * 0.90
});
});
Running this now produces: Cannot find module './calculateTotal' — correct RED state.
Write the minimum code needed to pass the test. Don't add anything extra.
// calculateTotal.js
function calculateTotal(items) {
const total = items.reduce((sum, item) => sum + item.price, 0);
return total > 100 ? total * 0.9 : total;
}
module.exports = { calculateTotal };
Run the test — it passes. GREEN achieved. Stop here; resist adding more logic.
With a passing test as your safety net, clean up the implementation. Run tests after every change.
// calculateTotal.js — refactored for clarity
const DISCOUNT_THRESHOLD = 100;
const DISCOUNT_RATE = 0.9;
function calculateTotal(items) {
const subtotal = items.reduce((sum, { price }) => sum + price, 0);
return subtotal > DISCOUNT_THRESHOLD ? subtotal * DISCOUNT_RATE : subtotal;
}
module.exports = { calculateTotal };
Test still passes — GREEN maintained. Constants now communicate intent.
Next requirement: apply a 15% discount when total exceeds 200.
RED — write the failing test first:
it('should apply 15% discount when total exceeds 200', () => {
const items = [{ price: 110 }, { price: 110 }]; // total = 220
expect(calculateTotal(items)).toBe(187); // 220 * 0.85
});
GREEN — extend the implementation minimally:
function calculateTotal(items) {
const subtotal = items.reduce((sum, { price }) => sum + price, 0);
if (subtotal > 200) return subtotal * 0.85;
if (subtotal > 100) return subtotal * 0.9;
return subtotal;
}
REFACTOR — remove duplication with a tiered structure:
const DISCOUNT_TIERS = [
{ threshold: 200, rate: 0.85 },
{ threshold: 100, rate: 0.9 },
];
function calculateTotal(items) {
const subtotal = items.reduce((sum, { price }) => sum + price, 0);
const tier = DISCOUNT_TIERS.find(({ threshold }) => subtotal > threshold);
return tier ? subtotal * tier.rate : subtotal;
}
Both tests pass — ready for the next cycle.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Create or update AgentSkills. Use when designing, structuring, or packaging skills with scripts, references, and assets.
Set up and use 1Password CLI (op). Use when installing the CLI, enabling desktop app integration, signing in (single or multi-account), or reading/injecting/running secrets via op.
CLI to manage emails via IMAP/SMTP. Use `himalaya` to list, read, write, reply, forward, search, and organize emails from the terminal. Supports multiple accounts and message composition with MML (MIME Meta Language).