fix(build): compose markdown `preConfig` hook when extending configs

x-ref: #5205

Co-authored-by: Jonathan Doughty <jonathandoughty04@gmail.com>

__tests__/unit/node/config.test.ts

@@ -1,15 +1,18 @@
 import type { MarkdownItAsync } from 'markdown-it-async'
-import { mergeConfig } from 'node/config'
+import { mergeConfig, type UserConfig } from 'node/config'
 
 describe('node/config', () => {
-  test('merges markdown config hooks from extended configs', async () => {
+  test('merges markdown hooks from extended configs', async () => {
     const calls: string[] = []
     const md = {} as MarkdownItAsync
 
-    const merged = mergeConfig(
+    const merged = mergeConfig<UserConfig, UserConfig>(
       {
         markdown: {
           lineNumbers: true,
+          preConfig() {
+            calls.push('base-pre')
+          },
           config() {
             calls.push('base')
           }
@@ -20,6 +23,9 @@ describe('node/config', () => {
           attrs: {
             allowedAttributes: ['id']
           },
+          async preConfig() {
+            calls.push('extended-pre')
+          },
           async config() {
             calls.push('extended')
           }
@@ -32,8 +38,36 @@ describe('node/config', () => {
       allowedAttributes: ['id']
     })
 
+    await merged.markdown?.preConfig?.(md)
+    await merged.markdown?.config?.(md)
+
+    expect(calls).toEqual(['base-pre', 'extended-pre', 'base', 'extended'])
+  })
+
+  test('keeps one-sided markdown hooks when the other config omits them', async () => {
+    const calls: string[] = []
+    const md = {} as MarkdownItAsync
+
+    const merged = mergeConfig<UserConfig, UserConfig>(
+      {
+        markdown: {
+          preConfig() {
+            calls.push('base-pre')
+          }
+        }
+      },
+      {
+        markdown: {
+          config() {
+            calls.push('extended')
+          }
+        }
+      }
+    )
+
+    await merged.markdown?.preConfig?.(md)
     await merged.markdown?.config?.(md)
 
-    expect(calls).toEqual(['base', 'extended'])
+    expect(calls).toEqual(['base-pre', 'extended'])
   })
 })

src/node/config.ts

@@ -332,15 +332,22 @@ export function mergeConfig<A extends object, B extends object>(
 
 function mergeMarkdownConfig(a: MarkdownOptions, b: MarkdownOptions) {
   const merged = mergeConfig(a, b, false)
-  const baseConfig = a.config
+  merged.preConfig = mergeMarkdownHooks(a.preConfig, b.preConfig)
-  const extendedConfig = b.config
+  merged.config = mergeMarkdownHooks(a.config, b.config)
-  if (baseConfig && extendedConfig) {
+  return merged
-    merged.config = async (md) => {
+}
-      await baseConfig(md)
+
-      await extendedConfig(md)
+function mergeMarkdownHooks(
+  base: MarkdownOptions['config'],
+  extended: MarkdownOptions['config']
+): MarkdownOptions['config'] {
+  if (!base || !extended) {
+    return base ?? extended
   }
+  return async (md) => {
+    await base(md)
+    await extended(md)
   }
-  return merged
 }
 
 export async function resolveSiteData(