1 files changed, 86 insertions, 0 deletions
diff --git a/packages/markdown/remark/src/frontmatter.ts b/packages/markdown/remark/src/frontmatter.ts
new file mode 100644
index 000000000..16c103f73
--- /dev/null
+++ b/packages/markdown/remark/src/frontmatter.ts
@@ -0,0 +1,86 @@
+import yaml from 'js-yaml';
+import * as toml from 'smol-toml';
+
+export function isFrontmatterValid(frontmatter: Record<string, any>) {
+	try {
+		// ensure frontmatter is JSON-serializable
+		JSON.stringify(frontmatter);
+	} catch {
+		return false;
+	}
+	return typeof frontmatter === 'object' && frontmatter !== null;
+}
+
+// Capture frontmatter wrapped with `---` or `+++`, including any characters and new lines within it.
+// Only capture if `---` or `+++` exists near the top of the file, including:
+// 1. Start of file (including if has BOM encoding)
+// 2. Start of file with any whitespace (but `---` or `+++` must still start on a new line)
+const frontmatterRE = /(?:^\uFEFF?|^\s*\n)(?:---|\+\+\+)([\s\S]*?\n)(?:---|\+\+\+)/;
+const frontmatterTypeRE = /(?:^\uFEFF?|^\s*\n)(---|\+\+\+)/;
+export function extractFrontmatter(code: string): string | undefined {
+	return frontmatterRE.exec(code)?.[1];
+}
+
+function getFrontmatterParser(code: string): [string, (str: string) => unknown] {
+	return frontmatterTypeRE.exec(code)?.[1] === '+++' ? ['+++', toml.parse] : ['---', yaml.load];
+}
+export interface ParseFrontmatterOptions {
+	/**
+	 * How the frontmatter should be handled in the returned `content` string.
+	 * - `preserve`: Keep the frontmatter.
+	 * - `remove`: Remove the frontmatter.
+	 * - `empty-with-spaces`: Replace the frontmatter with empty spaces. (preserves sourcemap line/col/offset)
+	 * - `empty-with-lines`: Replace the frontmatter with empty line breaks. (preserves sourcemap line/col)
+	 *
+	 * @default 'remove'
+	 */
+	frontmatter: 'preserve' | 'remove' | 'empty-with-spaces' | 'empty-with-lines';
+}
+
+export interface ParseFrontmatterResult {
+	frontmatter: Record<string, any>;
+	rawFrontmatter: string;
+	content: string;
+}
+
+export function parseFrontmatter(
+	code: string,
+	options?: ParseFrontmatterOptions,
+): ParseFrontmatterResult {
+	const rawFrontmatter = extractFrontmatter(code);
+
+	if (rawFrontmatter == null) {
+		return { frontmatter: {}, rawFrontmatter: '', content: code };
+	}
+	const [delims, parser] = getFrontmatterParser(code);
+	const parsed = parser(rawFrontmatter);
+	const frontmatter = (parsed && typeof parsed === 'object' ? parsed : {}) as Record<string, any>;
+
+	let content: string;
+	switch (options?.frontmatter ?? 'remove') {
+		case 'preserve':
+			content = code;
+			break;
+		case 'remove':
+			content = code.replace(`${delims}${rawFrontmatter}${delims}`, '');
+			break;
+		case 'empty-with-spaces':
+			content = code.replace(
+				`${delims}${rawFrontmatter}${delims}`,
+				`   ${rawFrontmatter.replace(/[^\r\n]/g, ' ')}   `,
+			);
+			break;
+		case 'empty-with-lines':
+			content = code.replace(
+				`${delims}${rawFrontmatter}${delims}`,
+				rawFrontmatter.replace(/[^\r\n]/g, ''),
+			);
+			break;
+	}
+
+	return {
+		frontmatter,
+		rawFrontmatter,
+		content,
+	};
+}