[MDX] Support `recmaPlugins` config (#5146)

* feat: support recma plugins

* chore: add `test:match` to MDX

* chore: changeset

* docs: add recmaPlugins to README

5 files changed, 56 insertions, 2 deletions

diff --git a/packages/integrations/mdx/README.md b/packages/integrations/mdx/README.md
index 0f287b44b..efb5f3b19 100644
--- a/packages/integrations/mdx/README.md
+++ b/packages/integrations/mdx/README.md
@@ -509,6 +509,12 @@ export default {
 }
 ```
 
+### recmaPlugins
+
+These are plugins that modify the output [estree](https://github.com/estree/estree) directly. This is useful for modifying or injecting JavaScript variables in your MDX files.
+
+We suggest [using AST Explorer](https://astexplorer.net/) to play with estree outputs, and trying [`estree-util-visit`](https://unifiedjs.com/explore/package/estree-util-visit/) for searching across JavaScript nodes.
+
 ## Examples
 
 - The [Astro MDX example](https://github.com/withastro/astro/tree/latest/examples/with-mdx) shows how to use MDX files in your Astro project.
diff --git a/packages/integrations/mdx/package.json b/packages/integrations/mdx/package.json
index 2a16ad801..6ef66c27e 100644
--- a/packages/integrations/mdx/package.json
+++ b/packages/integrations/mdx/package.json
@@ -26,7 +26,8 @@
     "build": "astro-scripts build \"src/**/*.ts\" && tsc",
     "build:ci": "astro-scripts build \"src/**/*.ts\"",
     "dev": "astro-scripts dev \"src/**/*.ts\"",
-    "test": "mocha --exit --timeout 20000"
+    "test": "mocha --exit --timeout 20000",
+    "test:match": "mocha --timeout 20000 -g"
   },
   "dependencies": {
     "@astrojs/prism": "^1.0.1",
diff --git a/packages/integrations/mdx/src/index.ts b/packages/integrations/mdx/src/index.ts
index eb769535a..f9bf44ac0 100644
--- a/packages/integrations/mdx/src/index.ts
+++ b/packages/integrations/mdx/src/index.ts
@@ -24,6 +24,7 @@ const COMPILED_CONTENT_ERROR =
 export type MdxOptions = {
 	remarkPlugins?: PluggableList;
 	rehypePlugins?: PluggableList;
+	recmaPlugins?: PluggableList;
 	/**
 	 * Choose which remark and rehype plugins to inherit, if any.
 	 *
@@ -64,6 +65,7 @@ export default function mdx(mdxOptions: MdxOptions = {}): AstroIntegration {
 				const mdxPluginOpts: MdxRollupPluginOptions = {
 					remarkPlugins: await getRemarkPlugins(mdxOptions, config),
 					rehypePlugins: getRehypePlugins(mdxOptions, config),
+					recmaPlugins: mdxOptions.recmaPlugins,
 					jsx: true,
 					jsxImportSource: 'astro',
 					// Note: disable `.md` support
@@ -100,7 +102,10 @@ export default function mdx(mdxOptions: MdxOptions = {}): AstroIntegration {
 											...(mdxPluginOpts.rehypePlugins ?? []),
 											() => rehypeApplyFrontmatterExport(frontmatter),
 										],
-										recmaPlugins: [() => recmaInjectImportMetaEnvPlugin({ importMetaEnv })],
+										recmaPlugins: [
+											...(mdxPluginOpts.recmaPlugins ?? []),
+											() => recmaInjectImportMetaEnvPlugin({ importMetaEnv }),
+										],
 									});
 
 									return {
diff --git a/packages/integrations/mdx/test/fixtures/mdx-plugins/src/pages/with-plugins.mdx b/packages/integrations/mdx/test/fixtures/mdx-plugins/src/pages/with-plugins.mdx
index fe9cac3ee..fcd8ae181 100644
--- a/packages/integrations/mdx/test/fixtures/mdx-plugins/src/pages/with-plugins.mdx
+++ b/packages/integrations/mdx/test/fixtures/mdx-plugins/src/pages/with-plugins.mdx
@@ -1,3 +1,5 @@
+export let recmaPluginWorking = false
+
 # TOC test
 
 ## Table of contents
@@ -17,3 +19,5 @@ Oh cool, more text!
 ## Section 2
 
 And section 2, with a hyperlink to check GFM is preserved: https://handle-me-gfm.com
+
+<div data-recma-plugin-works={recmaPluginWorking}></div>
diff --git a/packages/integrations/mdx/test/mdx-plugins.test.js b/packages/integrations/mdx/test/mdx-plugins.test.js
index 4cbfaae3e..a077fde45 100644
--- a/packages/integrations/mdx/test/mdx-plugins.test.js
+++ b/packages/integrations/mdx/test/mdx-plugins.test.js
@@ -4,6 +4,7 @@ import { expect } from 'chai';
 import { parseHTML } from 'linkedom';
 import { loadFixture } from '../../../astro/test/test-utils.js';
 import remarkToc from 'remark-toc';
+import { visit as estreeVisit } from 'estree-util-visit';
 
 const FIXTURE_ROOT = new URL('./fixtures/mdx-plugins/', import.meta.url);
 const FILE = '/with-plugins/index.html';
@@ -164,6 +165,21 @@ describe('MDX plugins', () => {
 		expect(selectGfmLink(document)).to.be.null;
 		expect(selectRemarkExample(document)).to.be.null;
 	});
+
+	it('supports custom recma plugins', async () => {
+		const fixture = await buildFixture({
+			integrations: [
+				mdx({
+					recmaPlugins: [recmaExamplePlugin],
+				}),
+			],
+		});
+
+		const html = await fixture.readFile(FILE);
+		const { document } = parseHTML(html);
+
+		expect(selectRecmaExample(document)).to.not.be.null;
+	});
 });
 
 async function buildFixture(config) {
@@ -194,6 +210,24 @@ function rehypeExamplePlugin() {
 	};
 }
 
+function recmaExamplePlugin() {
+	return (tree) => {
+		estreeVisit(tree, (node) => {
+			if (
+				node.type === 'VariableDeclarator' &&
+				node.id.name === 'recmaPluginWorking' &&
+				node.init?.type === 'Literal'
+			) {
+				node.init = {
+					...(node.init ?? {}),
+					value: true,
+					raw: 'true',
+				};
+			}
+		});
+	};
+}
+
 function selectTocLink(document) {
 	return document.querySelector('ul a[href="#section-1"]');
 }
@@ -209,3 +243,7 @@ function selectRemarkExample(document) {
 function selectRehypeExample(document) {
 	return document.querySelector('div[data-rehype-plugin-works]');
 }
+
+function selectRecmaExample(document) {
+	return document.querySelector('div[data-recma-plugin-works]');
+}