Github workflow to format files with Prettier on pull request

format-code-pull-request.yml

name: format-code-pull-request
on:
  - pull_request

jobs:
  format-source-code:
    # Check if the PR is not from a fork
    if: github.event.pull_request.head.repo.full_name == github.repository
    runs-on: ubuntu-latest
    # only run if there are changed files
    steps:
      - name: Checkout repository
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
          ref: ${{ github.event.pull_request.head.sha }}
      - name: Install NodeJS
        uses: actions/setup-node@v2
        with:
          node-version: "14"
          cache: 'npm'
      - name: Install dependencies
        run: npm ci
      - name: Get changed files
        id: changes
        run: |
          git diff -z --name-only --diff-filter=ACMRT origin/${{ github.event.pull_request.base.ref}}...origin/${{ github.event.pull_request.head.ref }} | xargs -0 -I {} npx prettier --write "{}"
      - name: Commit formatted files
        run: |
          git config --local user.email "$(git log --format='%ae' HEAD^!)"
          git config --local user.name "$(git log --format='%an' HEAD^!)"
          git add .
          if [ -z "$(git status --porcelain)" ]; then
            exit 0
          fi
          git commit -m "chore: format source code with Prettier"
          git push origin HEAD:${{ github.event.pull_request.head.ref }}

Note: it formats only the files changed in the pull-request, so it is safe to use on bigger projects where none of the files have been formatted previously.