[fix] upgrade the frontmatter stringify-handling functions

lib/frontmatter.ts

@@ -0,0 +1,38 @@
+/**
+ * Checks if the object contains only valid types.
+ */
+function isValidFrontmatterObject(obj: Record<string, any>): boolean {
+  return Object.values(obj).every(
+    (value) =>
+      value == null || // null or undefined
+      typeof value === "string" ||
+      typeof value === "boolean" ||
+      typeof value === "number" ||
+      (Array.isArray(value) && value.every((item) => typeof item === "string")), // valid string array
+  );
+}
+
+/**
+ * Converts a valid object into YAML-like frontmatter string.
+ * Throws error if object contains invalid types.
+ */
+export function stringifyFrontmatter(obj: Record<string, any>): string {
+  if (!isValidFrontmatterObject(obj)) {
+    throw new Error("Invalid frontmatter");
+  }
+
+  return `---\n${Object.entries(obj)
+    .map(([key, value]) => {
+      if (value == null) {
+        return `${key}: null`; // Handle null values explicitly
+      }
+      if (Array.isArray(value)) {
+        return `${key}: ${JSON.stringify(value)}`; // Handle arrays
+      }
+      if (typeof value === "string") {
+        return `${key}: ${JSON.stringify(value)}`; // Wrap strings with quotes
+      }
+      return `${key}: ${value}`; // Handle numbers and booleans
+    })
+    .join("\n")}\n---`;
+}