1 files changed, 41 insertions, 0 deletions
diff --git a/packages/markdown/remark/src/frontmatter-injection.ts b/packages/markdown/remark/src/frontmatter-injection.ts
new file mode 100644
index 000000000..921d01297
--- /dev/null
+++ b/packages/markdown/remark/src/frontmatter-injection.ts
@@ -0,0 +1,41 @@
+import type { Data, VFile } from 'vfile';
+import type { MarkdownAstroData } from './types.js';
+
+function isValidAstroData(obj: unknown): obj is MarkdownAstroData {
+	if (typeof obj === 'object' && obj !== null && obj.hasOwnProperty('frontmatter')) {
+		const { frontmatter } = obj as any;
+		try {
+			// ensure frontmatter is JSON-serializable
+			JSON.stringify(frontmatter);
+		} catch {
+			return false;
+		}
+		return typeof frontmatter === 'object' && frontmatter !== null;
+	}
+	return false;
+}
+
+export class InvalidAstroDataError extends TypeError {}
+
+export function safelyGetAstroData(vfileData: Data): MarkdownAstroData | InvalidAstroDataError {
+	const { astro } = vfileData;
+
+	if (!astro || !isValidAstroData(astro)) {
+		return new InvalidAstroDataError();
+	}
+
+	return astro;
+}
+
+export function toRemarkInitializeAstroData({
+	userFrontmatter,
+}: {
+	userFrontmatter: Record<string, any>;
+}) {
+	return () =>
+		function (tree: any, vfile: VFile) {
+			if (!vfile.data.astro) {
+				vfile.data.astro = { frontmatter: userFrontmatter };
+			}
+		};
+}