Day 3 — Matrix Testing: Prove Your Code Works on Node 18, 20, and 22

My project runs on Node 22 locally.

But what if someone uses it on Node 18? Or the server runs Node 20?

Matrix testing runs your tests across multiple Node versions simultaneously — three parallel jobs, one for each version.

If there is a compatibility issue, you find out before it reaches production.

The workflow

```yaml
name: Matrix CI

on:
  push:
    branches: [ master, feature/github-actions-practice ]
  pull_request:
    branches: [ master ]
  workflow_dispatch:

jobs:
  test:
    runs-on: ubuntu-latest
    
    strategy:
      matrix:
        node-version: [18, 20, 22]  # Test on 3 Node versions!
    
    steps:
    - name: Checkout code
      uses: actions/checkout@v4
    
    - name: Setup Node.js ${{ matrix.node-version }}
      uses: actions/setup-node@v4
      with:
        node-version: ${{ matrix.node-version }}
    
    - name: Install dependencies
      run: npm ci
    
    - name: Run tests
      run: npm test -- --passWithNoTests --watchAll=false
    
    - name: Build project
      run: npm run build
    
    - name: Display Node version
      run: |
        echo "Testing on Node.js ${{ matrix.node-version }}"
        node…