Frontmatter injection for MD and MDX

.changeset/cool-crabs-trade.md

@@ -0,0 +1,7 @@
+---
+'astro': minor
+'@astrojs/mdx': minor
+'@astrojs/markdown-remark': patch
+---
+
+Support frontmatter injection for MD and MDX using remark and rehype plugins

packages/astro/src/@types/astro.ts

@@ -1110,3 +1110,5 @@ export interface SSRResult {
 	response: ResponseInit;
 	_metadata: SSRMetadata;
 }
+
+export type MarkdownAstroData = { frontmatter: object };

packages/astro/src/vite-plugin-markdown/index.ts

@@ -7,7 +7,7 @@ import { collectErrorMetadata } from '../core/errors.js';
 import type { LogOptions } from '../core/logger/core.js';
 import { warn } from '../core/logger/core.js';
 import type { PluginMetadata } from '../vite-plugin-astro/types.js';
-import { getFileInfo } from '../vite-plugin-utils/index.js';
+import { getFileInfo, safelyGetAstroData } from '../vite-plugin-utils/index.js';
 
 interface AstroPluginOptions {
 	config: AstroConfig;
@@ -44,7 +44,14 @@ export default function markdown({ config, logging }: AstroPluginOptions): Plugi
 
 				const html = renderResult.code;
 				const { headings } = renderResult.metadata;
-				const frontmatter = { ...raw.data, url: fileUrl, file: fileId } as any;
+				const { frontmatter: injectedFrontmatter } = safelyGetAstroData(renderResult.vfile.data);
+				const frontmatter = {
+					...injectedFrontmatter,
+					...raw.data,
+					url: fileUrl,
+					file: fileId,
+				} as any;
+
 				const { layout } = frontmatter;
 
 				if (frontmatter.setup) {
@@ -94,6 +101,7 @@ export default function markdown({ config, logging }: AstroPluginOptions): Plugi
 				}
 				export default Content;
 				`);
+
 				return {
 					code,
 					meta: {

packages/astro/src/vite-plugin-utils/index.ts

@@ -1,4 +1,5 @@
-import type { AstroConfig } from '../@types/astro';
+import { Data } from 'vfile';
+import type { AstroConfig, MarkdownAstroData } from '../@types/astro';
 import { appendForwardSlash } from '../core/path.js';
 
 export function getFileInfo(id: string, config: AstroConfig) {
@@ -15,3 +16,30 @@ export function getFileInfo(id: string, config: AstroConfig) {
 	}
 	return { fileId, fileUrl };
 }
+
+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 function safelyGetAstroData(vfileData: Data): MarkdownAstroData {
+	const { astro } = vfileData;
+
+	if (!astro) return { frontmatter: {} };
+	if (!isValidAstroData(astro)) {
+		throw Error(
+			`[Markdown] A remark or rehype plugin tried to add invalid frontmatter. Ensure "astro.frontmatter" is a JSON object!`
+		);
+	}
+
+	return astro;
+}

packages/astro/test/astro-markdown-frontmatter-injection.test.js

@@ -0,0 +1,40 @@
+import { expect } from 'chai';
+import { loadFixture } from './test-utils.js';
+
+const FIXTURE_ROOT = './fixtures/astro-markdown-frontmatter-injection/';
+
+describe('Astro Markdown - frontmatter injection', () => {
+	let fixture;
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: FIXTURE_ROOT,
+		});
+		await fixture.build();
+	});
+
+	it('remark supports custom vfile data - get title', async () => {
+		const frontmatterByPage = JSON.parse(await fixture.readFile('/glob.json'));
+		const titles = frontmatterByPage.map((frontmatter = {}) => frontmatter.title);
+		expect(titles).to.contain('Page 1');
+		expect(titles).to.contain('Page 2');
+	});
+
+	it('rehype supports custom vfile data - reading time', async () => {
+		const frontmatterByPage = JSON.parse(await fixture.readFile('/glob.json'));
+		const readingTimes = frontmatterByPage.map((frontmatter = {}) => frontmatter.injectedReadingTime);
+		expect(readingTimes.length).to.be.greaterThan(0);
+		for (let readingTime of readingTimes) {
+			expect(readingTime).to.not.be.null;
+			expect(readingTime.text).match(/^\d+ min read/);
+		}
+	});
+
+	it('overrides injected frontmatter with user frontmatter', async () => {
+		const frontmatterByPage = JSON.parse(await fixture.readFile('/glob.json'));
+		const readingTimes = frontmatterByPage.map((frontmatter = {}) => frontmatter.injectedReadingTime?.text);
+		const titles = frontmatterByPage.map((frontmatter = {}) => frontmatter.title);
+		expect(titles).to.contain('Overridden title');
+		expect(readingTimes).to.contain('1000 min read');
+	});
+});

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/astro.config.mjs

@@ -0,0 +1,11 @@
+import { defineConfig } from 'astro/config';
+import { rehypeReadingTime, remarkTitle } from './src/markdown-plugins.mjs'
+
+// https://astro.build/config
+export default defineConfig({
+  site: 'https://astro.build/',
+	markdown: {
+		remarkPlugins: [remarkTitle],
+		rehypePlugins: [rehypeReadingTime],
+	}
+});

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "@test/astro-markdown-frontmatter-injection",
+  "version": "0.0.0",
+  "private": true,
+  "dependencies": {
+    "astro": "workspace:*",
+    "mdast-util-to-string": "^3.1.0",
+    "reading-time": "^1.5.0",
+    "unist-util-visit": "^4.1.0"
+  }
+}

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/src/markdown-plugins.mjs

@@ -0,0 +1,20 @@
+import getReadingTime from 'reading-time';
+import { toString } from 'mdast-util-to-string';
+import { visit } from 'unist-util-visit';
+
+export function rehypeReadingTime() {
+	return function (tree, { data }) {
+		const readingTime = getReadingTime(toString(tree));
+		data.astro.frontmatter.injectedReadingTime = readingTime;
+	};
+}
+
+export function remarkTitle() {
+	return function (tree, { data }) {
+		visit(tree, ['heading'], (node) => {
+			if (node.depth === 1) {
+				data.astro.frontmatter.title = toString(node.children);
+			}
+		});
+	};
+}

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/src/pages/glob.json.js

@@ -0,0 +1,6 @@
+export async function get() {
+	const docs = await import.meta.glob('./*.md', { eager: true });
+	return {
+		body: JSON.stringify(Object.values(docs).map(doc => doc.frontmatter)),
+	}
+}

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/src/pages/page-1.md

@@ -0,0 +1,3 @@
+# Page 1
+
+Look at that!

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/src/pages/page-2.md

@@ -0,0 +1,19 @@
+# Page 2
+
+## Table of contents
+
+## Section 1
+
+Some text!
+
+### Subsection 1
+
+Some subsection test!
+
+### Subsection 2
+
+Oh cool, more text!
+
+## Section 2
+
+More content

packages/astro/test/fixtures/astro-markdown-frontmatter-injection/src/pages/with-overrides.md

@@ -0,0 +1,7 @@
+---
+title: 'Overridden title'
+injectedReadingTime: 
+  text: '1000 min read'
+---
+
+# Working!

packages/astro/test/fixtures/astro-markdown/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@test/astro-markdown-md-mode",
+  "name": "@test/astro-markdown",
   "version": "0.0.0",
   "private": true,
   "dependencies": {

packages/integrations/mdx/package.json

@@ -43,7 +43,8 @@
     "remark-shiki-twoslash": "^3.1.0",
     "remark-smartypants": "^2.0.0",
     "shiki": "^0.10.1",
-    "unist-util-visit": "^4.1.0"
+    "unist-util-visit": "^4.1.0",
+    "vfile": "^5.3.2"
   },
   "devDependencies": {
     "@types/chai": "^4.3.1",

packages/integrations/mdx/src/astro-data-utils.ts

@@ -0,0 +1,82 @@
+import { name as isValidIdentifierName } from 'estree-util-is-identifier-name';
+import type { VFile } from 'vfile';
+import type { MdxjsEsm } from 'mdast-util-mdx';
+import type { MarkdownAstroData } from 'astro';
+import type { Data } from 'vfile';
+import { jsToTreeNode } from './utils.js';
+
+export function remarkInitializeAstroData() {
+	return function (tree: any, vfile: VFile) {
+		if (!vfile.data.astro) {
+			vfile.data.astro = { frontmatter: {} };
+		}
+	};
+}
+
+export function rehypeApplyFrontmatterExport(
+	pageFrontmatter: Record<string, any>,
+	exportName = 'frontmatter'
+) {
+	return function (tree: any, vfile: VFile) {
+		if (!isValidIdentifierName(exportName)) {
+			throw new Error(
+				`[MDX] ${JSON.stringify(
+					exportName
+				)} is not a valid frontmatter export name! Make sure "frontmatterOptions.name" could be used as a JS export (i.e. "export const frontmatterName = ...")`
+			);
+		}
+		const { frontmatter: injectedFrontmatter } = safelyGetAstroData(vfile.data);
+		const frontmatter = { ...injectedFrontmatter, ...pageFrontmatter };
+		let exportNodes: MdxjsEsm[] = [];
+		if (!exportName) {
+			exportNodes = Object.entries(frontmatter).map(([k, v]) => {
+				if (!isValidIdentifierName(k)) {
+					throw new Error(
+						`[MDX] A remark or rehype plugin tried to inject ${JSON.stringify(
+							k
+						)} as a top-level export, which is not a valid export name.`
+					);
+				}
+				return jsToTreeNode(`export const ${k} = ${JSON.stringify(v)};`);
+			});
+		} else {
+			exportNodes = [jsToTreeNode(`export const ${exportName} = ${JSON.stringify(frontmatter)};`)];
+		}
+		tree.children = exportNodes.concat(tree.children);
+	};
+}
+
+/**
+ * Copied from markdown utils
+ * @see "vite-plugin-utils"
+ */
+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;
+}
+
+/**
+ * Copied from markdown utils
+ * @see "vite-plugin-utils"
+ */
+export function safelyGetAstroData(vfileData: Data): MarkdownAstroData {
+	const { astro } = vfileData;
+
+	if (!astro) return { frontmatter: {} };
+	if (!isValidAstroData(astro)) {
+		throw Error(
+			`[MDX] A remark or rehype plugin tried to add invalid frontmatter. Ensure "astro.frontmatter" is a JSON object!`
+		);
+	}
+
+	return astro;
+}