refactor: centralize pre-commit file filtering

scripts/pre-commit/filter-staged-files.mjs

@@ -0,0 +1,39 @@
+#!/usr/bin/env node
+import path from "node:path";
+
+/**
+ * Prints selected files as NUL-delimited tokens to stdout.
+ *
+ * Usage:
+ *   node scripts/pre-commit/filter-staged-files.mjs lint -- <files...>
+ *   node scripts/pre-commit/filter-staged-files.mjs format -- <files...>
+ *
+ * Keep this dependency-free: the pre-commit hook runs in many environments.
+ */
+
+const mode = process.argv[2];
+const rawArgs = process.argv.slice(3);
+const files = rawArgs[0] === "--" ? rawArgs.slice(1) : rawArgs;
+
+if (mode !== "lint" && mode !== "format") {
+  process.stderr.write("usage: filter-staged-files.mjs <lint|format> -- <files...>\n");
+  process.exit(2);
+}
+
+const lintExts = new Set([".ts", ".tsx", ".js", ".jsx", ".mjs", ".cjs"]);
+const formatExts = new Set([".ts", ".tsx", ".js", ".jsx", ".mjs", ".cjs", ".json", ".md", ".mdx"]);
+
+const shouldSelect = (filePath) => {
+  const ext = path.extname(filePath).toLowerCase();
+  if (mode === "lint") {
+    return lintExts.has(ext);
+  }
+  return formatExts.has(ext);
+};
+
+for (const file of files) {
+  if (shouldSelect(file)) {
+    process.stdout.write(file);
+    process.stdout.write("\0");
+  }
+}