feat(markdown): add support for `shiki` option `langAlias` (#12039)

* feat(shiki): add support for `langAlias`

* chore: apply feedback

* Update packages/markdown/remark/src/types.ts

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

* fix build

* Fix bug

* Apply suggestions from code review

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* Update .changeset/dirty-socks-sip.md

---------

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

6 files changed, 78 insertions, 4 deletions

diff --git a/.changeset/dirty-socks-sip.md b/.changeset/dirty-socks-sip.md
new file mode 100644
index 000000000..1784e2031
--- /dev/null
+++ b/.changeset/dirty-socks-sip.md
@@ -0,0 +1,34 @@
+---
+'@astrojs/markdown-remark': minor
+'astro': minor
+---
+
+Adds a `markdown.shikiConfig.langAlias` option that allows [aliasing a non-supported code language to a known language](https://shiki.style/guide/load-lang#custom-language-aliases). This is useful when the language of your code samples is not [a built-in Shiki language](https://shiki.style/languages), but you want your Markdown source to contain an accurate language while also displaying syntax highlighting.
+
+The following example configures Shiki to highlight `cjs` code blocks using the `javascript` syntax highlighter:
+
+```js
+import { defineConfig } from 'astro/config';
+
+export default defineConfig({
+  markdown: {
+    shikiConfig: {
+      langAlias: {
+        cjs: 'javascript',
+      },
+    },
+  },
+});
+```
+
+Then in your Markdown, you can use the alias as the language for a code block for syntax highlighting:
+
+````md
+```cjs
+'use strict';
+
+function commonJs() {
+  return 'I am a commonjs file';
+}
+```
+````
diff --git a/packages/astro/src/core/config/schema.ts b/packages/astro/src/core/config/schema.ts
index da70ead1a..27f703a10 100644
--- a/packages/astro/src/core/config/schema.ts
+++ b/packages/astro/src/core/config/schema.ts
@@ -313,6 +313,10 @@ export const AstroConfigSchema = z.object({
 							return langs;
 						})
 						.default([]),
+					langAlias: z
+						.record(z.string(), z.string())
+						.optional()
+						.default(ASTRO_CONFIG_DEFAULTS.markdown.shikiConfig.langAlias!),
 					theme: z
 						.enum(Object.keys(bundledThemes) as [BuiltinTheme, ...BuiltinTheme[]])
 						.or(z.custom<ShikiTheme>())
diff --git a/packages/markdown/remark/src/index.ts b/packages/markdown/remark/src/index.ts
index 5849f5799..a9ae7ed59 100644
--- a/packages/markdown/remark/src/index.ts
+++ b/packages/markdown/remark/src/index.ts
@@ -37,6 +37,7 @@ export const markdownConfigDefaults: Required<AstroMarkdownOptions> = {
 		themes: {},
 		wrap: false,
 		transformers: [],
+		langAlias: {},
 	},
 	remarkPlugins: [],
 	rehypePlugins: [],
diff --git a/packages/markdown/remark/src/shiki.ts b/packages/markdown/remark/src/shiki.ts
index 28f51c5e8..2f06ef9a9 100644
--- a/packages/markdown/remark/src/shiki.ts
+++ b/packages/markdown/remark/src/shiki.ts
@@ -45,24 +45,29 @@ export async function createShikiHighlighter({
 	defaultColor,
 	wrap = false,
 	transformers = [],
+	langAlias = {},
 }: ShikiConfig = {}): Promise<ShikiHighlighter> {
 	theme = theme === 'css-variables' ? cssVariablesTheme() : theme;
 
 	const highlighter = await getHighlighter({
 		langs: ['plaintext', ...langs],
+		langAlias,
 		themes: Object.values(themes).length ? Object.values(themes) : [theme],
 	});
 
 	return {
 		async highlight(code, lang = 'plaintext', options) {
+			const resolvedLang = langAlias[lang] ?? lang;
 			const loadedLanguages = highlighter.getLoadedLanguages();
 
-			if (!isSpecialLang(lang) && !loadedLanguages.includes(lang)) {
+			if (!isSpecialLang(lang) && !loadedLanguages.includes(resolvedLang)) {
 				try {
-					await highlighter.loadLanguage(lang as BundledLanguage);
+					await highlighter.loadLanguage(resolvedLang as BundledLanguage);
 				} catch (_err) {
+					const langStr =
+						lang === resolvedLang ? `"${lang}"` : `"${lang}" (aliased to "${resolvedLang}")`;
 					console.warn(
-						`[Shiki] The language "${lang}" doesn't exist, falling back to "plaintext".`,
+						`[Shiki] The language ${langStr} doesn't exist, falling back to "plaintext".`,
 					);
 					lang = 'plaintext';
 				}
@@ -120,7 +125,7 @@ export async function createShikiHighlighter({
 							// Add "user-select: none;" for "+"/"-" diff symbols.
 							// Transform `<span class="line"><span style="...">+ something</span></span>
 							// into      `<span class="line"><span style="..."><span style="user-select: none;">+</span> something</span></span>`
-							if (lang === 'diff') {
+							if (resolvedLang === 'diff') {
 								const innerSpanNode = node.children[0];
 								const innerSpanTextNode =
 									innerSpanNode?.type === 'element' && innerSpanNode.children?.[0];
diff --git a/packages/markdown/remark/src/types.ts b/packages/markdown/remark/src/types.ts
index aa7b62c9a..d95676b55 100644
--- a/packages/markdown/remark/src/types.ts
+++ b/packages/markdown/remark/src/types.ts
@@ -3,6 +3,7 @@ import type * as mdast from 'mdast';
 import type { Options as RemarkRehypeOptions } from 'remark-rehype';
 import type {
 	BuiltinTheme,
+	HighlighterCoreOptions,
 	LanguageRegistration,
 	ShikiTransformer,
 	ThemeRegistration,
@@ -37,6 +38,7 @@ export type ThemePresets = BuiltinTheme | 'css-variables';
 
 export interface ShikiConfig {
 	langs?: LanguageRegistration[];
+	langAlias?: HighlighterCoreOptions['langAlias'];
 	theme?: ThemePresets | ThemeRegistration | ThemeRegistrationRaw;
 	themes?: Record<string, ThemePresets | ThemeRegistration | ThemeRegistrationRaw>;
 	defaultColor?: 'light' | 'dark' | string | false;
diff --git a/packages/markdown/remark/test/shiki.test.js b/packages/markdown/remark/test/shiki.test.js
index c3cb81370..ca17ab1d8 100644
--- a/packages/markdown/remark/test/shiki.test.js
+++ b/packages/markdown/remark/test/shiki.test.js
@@ -101,4 +101,32 @@ describe('shiki syntax highlighting', () => {
 		// Doesn't have `color` or `background-color` properties.
 		assert.doesNotMatch(code, /color:/);
 	});
+
+	it('the highlighter supports lang alias', async () => {
+		const highlighter = await createShikiHighlighter({
+			langAlias: {
+				cjs: 'javascript',
+			},
+		});
+
+		const html = await highlighter.highlight(`let test = "some string"`, 'cjs', {
+			attributes: { 'data-foo': 'bar', autofocus: true },
+		});
+
+		assert.match(html, /data-language="cjs"/);
+	});
+
+	it('the markdown processsor support lang alias', async () => {
+		const processor = await createMarkdownProcessor({
+			shikiConfig: {
+				langAlias: {
+					cjs: 'javascript',
+				},
+			},
+		});
+
+		const { code } = await processor.render('```cjs\nlet foo = "bar"\n```');
+
+		assert.match(code, /data-language="cjs"/);
+	});
 });