fix(rehype): handle promise rejections in lazy language loading (#1221)

Author: ambujvashistha

packages/rehype/src/core.ts

@@ -149,7 +149,21 @@ function rehypeShikiFromHighlighter(
       if (lazyLoad) {
         try {
           // passed language is checked in sync, promise `.catch()` wouldn't work
-          queue.push(highlighter.loadLanguage(lang).then(() => processNode(lang)))
+          queue.push(
+            highlighter.loadLanguage(lang)
+              .then(() => processNode(lang))
+              .catch((error) => {
+                if (fallbackLanguage) {
+                  processNode(fallbackLanguage)
+                }
+                else if (onError) {
+                  onError(error)
+                }
+                else {
+                  throw error
+                }
+              }),
+          )
         }
         catch (error) {
           if (fallbackLanguage)

packages/rehype/test/core.test.ts

@@ -123,3 +123,113 @@ it('run with lazy + fallback language', async () => {
 
   await expect(file.toString()).toMatchFileSnapshot('./fixtures/d.out.html')
 })
+
+it('lazy loading error handling with fallbackLanguage', async () => {
+  using highlighter = await createHighlighter({
+    themes: ['vitesse-light'],
+    langs: ['text'],
+  })
+
+  // Create a mock highlighter that fails to load a specific language
+  const mockHighlighter = {
+    ...highlighter,
+    loadLanguage: async (...langs: Parameters<typeof highlighter.loadLanguage>) => {
+      const lang = langs[0] as string
+      if (lang === 'nonexistent-lang') {
+        throw new Error(`Language 'nonexistent-lang' not found`)
+      }
+      return highlighter.loadLanguage(...langs)
+    },
+  }
+
+  const markdown = '```nonexistent-lang\nconst x = 1\n```'
+
+  const file = await unified()
+    .use(remarkParse)
+    .use(remarkRehype)
+    .use(rehypeShikiFromHighlighter, mockHighlighter, {
+      lazy: true,
+      theme: 'vitesse-light',
+      fallbackLanguage: 'text',
+    })
+    .use(rehypeStringify)
+    .process(markdown)
+
+  // Should use fallback language (text) instead of throwing
+  expect(file.toString()).toContain('<pre')
+  expect(file.toString()).toContain('<code')
+})
+
+it('lazy loading error handling with onError callback', async () => {
+  using highlighter = await createHighlighter({
+    themes: ['vitesse-light'],
+    langs: ['text'],
+  })
+
+  const errors: unknown[] = []
+  const onError = (error: unknown) => {
+    errors.push(error)
+  }
+
+  // Create a mock highlighter that fails to load a specific language
+  const mockHighlighter = {
+    ...highlighter,
+    loadLanguage: async (...langs: Parameters<typeof highlighter.loadLanguage>) => {
+      const lang = langs[0] as string
+      if (lang === 'failing-lang') {
+        throw new Error(`Language 'failing-lang' not found`)
+      }
+      return highlighter.loadLanguage(...langs)
+    },
+  }
+
+  const markdown = '```failing-lang\nconst x = 1\n```'
+
+  await unified()
+    .use(remarkParse)
+    .use(remarkRehype)
+    .use(rehypeShikiFromHighlighter, mockHighlighter, {
+      lazy: true,
+      theme: 'vitesse-light',
+      onError,
+    })
+    .use(rehypeStringify)
+    .process(markdown)
+
+  // onError should be called
+  expect(errors.length).toBeGreaterThan(0)
+  expect(errors[0]).toBeInstanceOf(Error)
+})
+
+it('lazy loading error handling throws when no fallback or onError', async () => {
+  using highlighter = await createHighlighter({
+    themes: ['vitesse-light'],
+    langs: ['text'],
+  })
+
+  // Create a mock highlighter that fails to load a specific language
+  const mockHighlighter = {
+    ...highlighter,
+    loadLanguage: async (...langs: Parameters<typeof highlighter.loadLanguage>) => {
+      const lang = langs[0] as string
+      if (lang === 'error-lang') {
+        throw new Error(`Language 'error-lang' not found`)
+      }
+      return highlighter.loadLanguage(...langs)
+    },
+  }
+
+  const markdown = '```error-lang\nconst x = 1\n```'
+
+  await expect(
+    unified()
+      .use(remarkParse)
+      .use(remarkRehype)
+      .use(rehypeShikiFromHighlighter, mockHighlighter, {
+        lazy: true,
+        theme: 'vitesse-light',
+      })
+      .use(rehypeStringify)
+      .process(markdown),
+  ).rejects.toThrow()
+})