Add preview mode checker fn

Author: ravewebdev

functions/isPreviewActive.js

@@ -0,0 +1,19 @@
+/**
+ * Check if preview mode is active for current post view.
+ *
+ * @param {object} post        Current post object.
+ * @param {object} previewData Post preview object.
+ * @return {boolean}           Whether preview mode is active for current view.
+ */
+export default function isPreviewActive(post, previewData) {
+  const previewId = previewData?.page?.id
+  const postId = post?.databaseId
+
+  // Not in preview mode if either ID is missing.
+  if (!previewId || !postId) {
+    return false
+  }
+
+  // Only in preview mode if IDs match.
+  return previewId === postId
+}