[MDX] Include `url` in glob result (#3981)

* deps: add es-module-lexer

* feat: inject url export on mdx files

* fix: apply url transform in prod

* test: page urls with overrides

* fix: revert test skips

* chore: changeset

* fix: add newline before export

2 files changed, 38 insertions, 6 deletions

diff --git a/packages/integrations/mdx/src/index.ts b/packages/integrations/mdx/src/index.ts
index bd3d50c86..7b4a24ff5 100644
--- a/packages/integrations/mdx/src/index.ts
+++ b/packages/integrations/mdx/src/index.ts
@@ -1,11 +1,13 @@
-import mdxPlugin from '@mdx-js/rollup';
 import type { AstroIntegration } from 'astro';
+import mdxPlugin from '@mdx-js/rollup';
+import { parse as parseESM } from 'es-module-lexer';
+import { getFileInfo } from './utils.js';
 
 export default function mdx(): AstroIntegration {
 	return {
 		name: '@astrojs/mdx',
 		hooks: {
-			'astro:config:setup': ({ updateConfig, addPageExtension, command }: any) => {
+			'astro:config:setup': ({ updateConfig, config, addPageExtension, command }: any) => {
 				addPageExtension('.mdx');
 				updateConfig({
 					vite: {
@@ -20,14 +22,23 @@ export default function mdx(): AstroIntegration {
 									mdExtensions: [],
 								}),
 							},
-							command === 'dev' && {
+							{
 								name: '@astrojs/mdx',
 								transform(code: string, id: string) {
 									if (!id.endsWith('.mdx')) return;
-									// TODO: decline HMR updates until we have a stable approach
-									return `${code}\nif (import.meta.hot) {
+									const [, moduleExports] = parseESM(code);
+
+									if (!moduleExports.includes('url')) {
+										const { fileUrl } = getFileInfo(id, config);
+										code += `\nexport const url = ${JSON.stringify(fileUrl)};`;
+									}
+									if (command === 'dev') {
+										// TODO: decline HMR updates until we have a stable approach
+										code += `\nif (import.meta.hot) {
 											import.meta.hot.decline();
-										}`;
+										}`
+									}
+									return code;
 								},
 							},
 						],
diff --git a/packages/integrations/mdx/src/utils.ts b/packages/integrations/mdx/src/utils.ts
new file mode 100644
index 000000000..1091e8511
--- /dev/null
+++ b/packages/integrations/mdx/src/utils.ts
@@ -0,0 +1,21 @@
+import type { AstroConfig } from 'astro';
+
+function appendForwardSlash(path: string) {
+	return path.endsWith('/') ? path : path + '/';
+}
+
+/** @see 'vite-plugin-utils' for source */
+export function getFileInfo(id: string, config: AstroConfig) {
+	const sitePathname = appendForwardSlash(
+		config.site ? new URL(config.base, config.site).pathname : config.base
+	);
+
+	const fileId = id.split('?')[0];
+	let fileUrl = fileId.includes('/pages/')
+		? fileId.replace(/^.*?\/pages\//, sitePathname).replace(/(\/index)?\.mdx$/, '')
+		: undefined;
+	if (fileUrl && config.trailingSlash === 'always') {
+		fileUrl = appendForwardSlash(fileUrl);
+	}
+	return { fileId, fileUrl };
+}