Danger check for unprettified JavaScript

This adds a Dangerfile which executes `prettier` to find out if someone
touched Frontend files and forgot to run it on their current branch.

1 files changed, 39 insertions, 0 deletions

diff --git a/danger/prettier/Dangerfile b/danger/prettier/Dangerfile
new file mode 100644
index 00000000000..86f9f6af475
--- /dev/null
+++ b/danger/prettier/Dangerfile
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+def get_prettier_files(files)
+  files.select do |file|
+    file.end_with?('.js', '.scss', '.vue')
+  end
+end
+
+prettier_candidates = get_prettier_files(git.added_files + git.modified_files)
+
+return if prettier_candidates.empty?
+
+unpretty = `node_modules/prettier/bin-prettier.js --list-different #{prettier_candidates.join(" ")}`
+             .split(/$/)
+             .map(&:strip)
+             .reject(&:empty?)
+
+return if unpretty.empty?
+
+warn 'This merge request changed frontend files without pretty printing them.'
+
+markdown(<<~MARKDOWN)
+  ## Pretty print Frontend files
+
+  The following files should have been pretty printed with `prettier`:
+
+  * #{unpretty.map { |path| "`#{path}`" }.join("\n* ")}
+
+  Please run
+
+  ```
+  node_modules/.bin/prettier --write \\
+  #{unpretty.map { |path| "  '#{path}'" }.join(" \\\n")}
+  ```
+
+  Also consider auto-formatting [on-save].
+
+  [on-save]: https://docs.gitlab.com/ee/development/new_fe_guide/style/prettier.html
+MARKDOWN