frontmatter-kit

Parse and inspect front matter with typed metadata, body ranges and readable diagnostics.

frontmatter-kit is a small clean-room toolkit for Markdown/content tools that need more than attributes + body. It parses front matter, exposes line/column ranges, reports malformed delimiters and metadata errors, and keeps the result easy to render in previews or docs dashboards.

Links: Demo · npm · GitHub

frontmatter-kit is intentionally not a drop-in replacement for every front matter package. It is best for tools that need to inspect content, explain what was parsed, and show users where metadata starts, ends or fails.

Package quality

• TypeScript types are generated from the source.
• ESM-only package with no runtime dependencies.
• Marked as side-effect free for bundlers.
• Tested on Node.js 20 and 22 with GitHub Actions.
• Works in Node.js, browsers, Vite apps and static docs tooling.

Install

npm install frontmatter-kit

Quick Start

import { parseFrontmatter } from "frontmatter-kit";

const result = parseFrontmatter(`---
title: Release notes
draft: false
tags:
  - docs
  - release
---
# Release notes

Short intro.
<!-- more -->
Full article.`);

result.attributes;
// { title: "Release notes", draft: false, tags: ["docs", "release"] }

result.body;
// "# Release notes\n\nShort intro.\n<!-- more -->\nFull article."

result.excerpt;
// "# Release notes\n\nShort intro."

result.ranges.matter?.start.line;
// 2

Why not just another front matter parser?

Use gray-matter when you need a broad, mature parser with many ecosystem integrations.

Use frontmatter-kit when you are building an inspector, editor, docs preview, static dashboard or content QA tool and need:

• predictable TypeScript result shapes;
• diagnostics instead of only thrown errors;
• line/column ranges for the opening marker, metadata block, closing marker, body and excerpt;
• a small browser-friendly package with no runtime dependencies.

Supported formats

| Marker | Format | | --- | --- | | --- | YAML-like key/value front matter | | ---yaml / ---yml | YAML-like key/value front matter | | ---json | JSON object front matter | | ---toml | TOML-like key/value front matter with dash delimiters | | +++ / +++toml | TOML-like key/value front matter with plus delimiters |

The YAML and TOML parsers are intentionally small. They cover common metadata shapes: strings, numbers, booleans, nulls, inline arrays, simple nested objects and simple lists. They are designed for content metadata, not for full YAML or TOML language coverage.

Unknown hints such as ---data are parsed as YAML-like metadata and reported with an UNKNOWN_LANGUAGE_HINT warning.

API

parseFrontmatter(source, options?)

Returns a structured result and never throws for normal syntax problems. Errors are reported in diagnostics.

import { parseFrontmatter } from "frontmatter-kit";

const result = parseFrontmatter(source, {
  excerptSeparator: "<!-- more -->"
});

if (result.diagnostics.some((diagnostic) => diagnostic.severity === "error")) {
  console.log(result.diagnostics);
}

Result shape:

type FrontmatterResult = {
  hasFrontmatter: boolean;
  attributes: Record<string, FrontmatterValue>;
  body: string;
  matter: string;
  language?: "yaml" | "json" | "toml";
  excerpt?: string;
  ranges: {
    opening?: FrontmatterRange;
    matter?: FrontmatterRange;
    closing?: FrontmatterRange;
    body: FrontmatterRange;
    excerpt?: FrontmatterRange;
  };
  diagnostics: FrontmatterDiagnostic[];
};

hasFrontmatter is true only when a complete opening marker, metadata block and closing marker were found. A document with an opening marker but no closing marker stays inspectable, but returns hasFrontmatter: false with a MISSING_CLOSING_DELIMITER diagnostic.

Options:

| Option | Default | Description | | --- | --- | --- | | language | inferred | Force yaml, json or toml. | | excerptSeparator | <!-- more --> | Marker used to derive excerpt. Set false to disable. |

parseFrontmatterOrThrow(source, options?)

Returns the same result as parseFrontmatter(), but throws FrontmatterParseError when an error diagnostic is present.

import { parseFrontmatterOrThrow } from "frontmatter-kit";

const result = parseFrontmatterOrThrow(source);

tryParseFrontmatter(source, options?)

Wraps unexpected runtime errors in a result object. Metadata syntax problems still appear in result.diagnostics, because parseFrontmatter() is designed to keep malformed documents inspectable.

import { tryParseFrontmatter } from "frontmatter-kit";

const parsed = tryParseFrontmatter(source);

if (parsed.ok) {
  console.log(parsed.result.attributes);
}

hasFrontmatter(source)

Checks whether a source string begins with a supported front matter marker.

import { hasFrontmatter } from "frontmatter-kit";

hasFrontmatter("---\ntitle: Hi\n---\nBody");
// true

stripFrontmatter(source, options?)

Returns only the document body. If the source has no front matter, the original document is returned.

import { stripFrontmatter } from "frontmatter-kit";

stripFrontmatter("---\ntitle: Notes\n---\n# Body");
// "# Body"

stringifyFrontmatter(attributes, body?, options?)

Serializes a small metadata object back to a front matter document. This is meant for simple metadata objects, not as a full YAML/TOML emitter.

import { stringifyFrontmatter } from "frontmatter-kit";

stringifyFrontmatter({ title: "Hello", draft: false }, "# Hello");

Diagnostics

Malformed documents stay inspectable:

const result = parseFrontmatter(`---
title: Missing close`);

result.diagnostics;
// [{ code: "MISSING_CLOSING_DELIMITER", severity: "error", ... }]

Each diagnostic can include a line/column range so editors and playgrounds can highlight the problem directly.

Common diagnostic codes:

| Code | Severity | Meaning | | --- | --- | --- | | MISSING_CLOSING_DELIMITER | error | The opening marker exists but no matching closing marker was found. | | INVALID_JSON_FRONTMATTER | error | JSON metadata could not be parsed. | | JSON_FRONTMATTER_NOT_OBJECT | error | JSON metadata parsed, but did not produce an object. | | UNKNOWN_LANGUAGE_HINT | warning | The marker had an unsupported language hint, so YAML-like parsing was used. | | INVALID_YAML_LINE | warning | A YAML-like line was ignored because it was not a key/value pair. | | INVALID_TOML_LINE | warning | A TOML-like line was ignored because it was not a key/value pair. |

Notes

• This package does not execute code and does not load external resources.
• The YAML/TOML support is intentionally pragmatic rather than exhaustive.
• Use gray-matter if you need a broad compatibility parser for many legacy front matter shapes.
• The implementation is clean-room and does not copy code from front-matter, gray-matter or related packages.

License

MPL-2.0