Outsource retrieval of changed files to reusable action

.github/actions/changed_files.yml

@@ -0,0 +1,39 @@
+name: "Get changed files"
+description: "Checks out the code and returns the filenames of files that have changed in the pull request"
+
+inputs:
+  file-extensions:
+    # for example: "\.rb$" or something like "\.js$|\.js.erb$"
+    description: "Regex expressions for grep to filter for specific files"
+    required: true
+
+outputs:
+  changed-files:
+    description: "A space-separated list of the files that have changed in the pull request"
+    value: ${{ steps.get-changed-files.outputs.files }}
+
+runs:
+  # For a small tutorial on GitHub composite actions, see
+  # https://docs.github.com/en/actions/creating-actions/creating-a-composite-action
+  using: "composite"
+  steps:
+    - name: "Checkout PR branch (with test merge-commit)"
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 2 # to also fetch parent of PR
+
+    # Adapted from this great comment [1]. Git diff adapted from [2].
+    # "|| test $? = 1;" is used to ignore the exit code of grep when no files
+    # are found matching the pattern. For the "three dots" ... syntax, see [3].
+    #
+    # Resources:
+    # number [1] being most important
+    # [1] https://github.com/actions/checkout/issues/520#issuecomment-1167205721
+    # [2] https://robertfaldo.medium.com/commands-to-run-rubocop-and-specs-you-changed-in-your-branch-e6d2f2e4110b
+    # [3] https://community.atlassian.com/t5/Bitbucket-questions/Git-diff-show-different-files-than-PR-Pull-Request/qaq-p/2331786
+    - name: Get changed files
+      id: get-changed-files
+      run: |
+        files_pretty=$(git diff --name-only --diff-filter=ACMR -r HEAD^1...HEAD | grep '${{inputs.file-extensions}}' || test $? = 1;)
+        printf "🎴 Changed files: \n$files_pretty"
+        echo "files=$(echo ${files_pretty} | xargs)" >> $GITHUB_ENV

.github/workflows/linter.yml

@@ -17,7 +17,7 @@ jobs:
     name: RuboCop (Ruby)
     runs-on: ubuntu-latest
     steps:
-      - name: 'Checkout PR branch (with test merge-commit)'
+      - name: Checkout PR branch (with test merge-commit)
         uses: actions/checkout@v4
         with:
           fetch-depth: 2 # to also fetch parent of PR
@@ -48,28 +48,22 @@ jobs:
       name: ESLint (JS)
       runs-on: ubuntu-latest
       steps:
-        - name: 'Checkout PR branch'
-          uses: actions/checkout@v4
+        - name: Checkout code & get changed files
+          id: eslint-changed
+          uses: ./.github/actions/changed_files
           with:
-            fetch-depth: 2 # to also fetch parent of PR
+            file-extensions: \.js$|\.js.erb$
 
-        # see notes above (in rubocop job)
-        - name: Get changed files
-          run: |
-            files=$(git diff --name-only --diff-filter=ACMR -r HEAD^1...HEAD | grep -e '\.js$' -e '\.js.erb$'|| test $? = 1;)
-            printf "🎴 Changed js files: \n$files"
-            echo "CHANGED_FILES=$(echo ${files} | xargs)" >> $GITHUB_ENV
-        
-        - name: 'Setup Node.js'
-          if: env.CHANGED_FILES != ''
+        - name: Setup Node.js
+          if: ${{ steps.eslint-changed.outputs.changed-files }} != ''
           uses: actions/setup-node@v4
           with:
             node-version: '20' # End of Life (EOL): April 2026
 
-        - name: 'Install dependencies'
-          if: env.CHANGED_FILES != ''
+        - name: Install dependencies
+          if: ${{ steps.eslint-changed.outputs.changed-files }} != ''
           run: yarn install
 
-        - name: 'Run ESLint'
-          if: env.CHANGED_FILES != ''
-          run: yarn run eslint --ignore-path .gitignore --max-warnings 0 $CHANGED_FILES
+        - name: Run ESLint
+          if: ${{ steps.eslint-changed.outputs.changed-files }} != ''
+          run: yarn run eslint --ignore-path .gitignore --max-warnings 0 ${{ steps.eslint-changed.outputs.changed-files }}