Testing & Deployment

Code that isn’t deployed doesn’t exist. Code without tests breaks silently.

Two skills define the difference between a developer who builds and a developer who ships.

Testing: knowing your code works before users find out it doesn’t. Deployment: getting it to the world reliably, repeatedly, automatically.

Unit Testing

Testing the smallest pieces

// sum.js
export const sum = (a, b) => a + b

// sum.test.js
import { sum } from './sum'

describe('sum function', () => {
  test('adds two positive numbers', () => {
    expect(sum(2, 3)).toBe(5)
  })

  test('handles negative numbers', () => {
    expect(sum(-1, 1)).toBe(0)
  })

  test('handles zero', () => {
    expect(sum(0, 0)).toBe(0)
  })
})

Run: jest or bun test

The output tells you what passed, what failed, and where.

What to Test

Priority	What to test	Why
High	Business logic functions	Wrong output = wrong data in production
High	Auth and permissions	Security failures are catastrophic
Medium	API endpoints	Catch regressions before deployment
Medium	Form validation	User-facing bugs erode trust
Low	UI components	High effort, low ROI unless shared library

Test the code whose failure costs the most. Not everything needs a test.

Environment Variables

Secrets

# .env (never commit this file)
DATABASE_URL=postgresql://user:pass@localhost:5432/mydb
JWT_SECRET=your-secret-key-here
OPENAI_API_KEY=sk-...

# .env.example (commit this — shows what variables are needed)
DATABASE_URL=
JWT_SECRET=
OPENAI_API_KEY=

// Access in code
const secret = process.env.JWT_SECRET

Rules:

.env is in .gitignore. Always.
.env.example is committed with empty values.
Production secrets go in your hosting platform’s env config — never in code.

Deploying

Zero-config deployment(Vercel) for Node and React

# Install Vercel CLI
npm i -g vercel

# Deploy from project root
vercel

# Deploy to production
vercel --prod

Vercel auto-detects React (Vite), Next.js, and Node.js projects.

Every push to main on GitHub triggers an automatic production deploy.

Every push to any other branch creates a preview URL for review.

Set environment variables in: Vercel Dashboard → Project → Settings → Environment Variables.

Docker Basics

Your app, packaged and portable

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

docker build -t my-app .
docker run -p 3000:3000 my-app

Docker solves “it works on my machine.”

The container runs identically on your laptop, a teammate’s machine, and a production server.

Essential for backend services that won’t be deployed to Vercel.

Next lesson: AI Tools & Workflow: build with intelligence.