Conditional steps with if expressions and job outputs

CI pipelines run expensive steps (e2e tests, deployments) even when they are not needed, for example when only documentation files change, or when the build step failed.

Use if conditions on steps and jobs. Read changed files or previous step results:

jobs:
  changes:
    runs-on: ubuntu-latest
    outputs:
      src: ${{ steps.filter.outputs.src }}
    steps:
      - uses: actions/checkout@v4
      - uses: dorny/paths-filter@v3
        id: filter
        with:
          filters: |
            src:
              - 'src/**'
              - 'package*.json'

  test:
    needs: changes
    if: needs.changes.outputs.src == 'true'
    runs-on: ubuntu-latest
    steps:
      - run: npm test

  deploy:
    needs: [test]
    if: success() && github.ref == 'refs/heads/main'
    runs-on: ubuntu-latest
    steps:
      - run: ./deploy.sh

Skipping jobs based on file changes saves runner minutes. Using success() and explicit branch checks prevents accidental deploys from PR branches or after test failures.