fix(useFetch): partial overwrite when {combination: 'overwrite'} (#4430)

Co-authored-by: Anthony Fu <github@antfu.me>

Author: pkc918

packages/core/useFetch/index.test.ts

@@ -807,4 +807,32 @@ describe.skipIf(isBelowNode18)('useFetch', () => {
       expect(options?.execute).toBeDefined()
     })
   })
+
+  it('should partial overwrite when combination is overwrite', async () => {
+    const useMyFetch = createFetch({
+      baseUrl: 'https://example.com',
+      combination: 'overwrite',
+      options: {
+        beforeFetch({ options }) {
+          options.headers = { ...options.headers, before: 'Global' }
+          return { options }
+        },
+        afterFetch(ctx) {
+          ctx.data.after = 'Global'
+          return ctx
+        },
+      },
+    })
+
+    const { data } = useMyFetch('test', {
+      beforeFetch({ options }) {
+        options.headers = { ...options.headers, before: 'Local' }
+      },
+    }).json()
+
+    await vi.waitFor(() => {
+      expect(fetchSpyHeaders()).toMatchObject({ before: 'Local' })
+      expect(data.value).toEqual(expect.objectContaining({ after: 'Global' }))
+    })
+  })
 })

packages/core/useFetch/index.ts

@@ -247,7 +247,13 @@ function combineCallbacks<T = any>(combination: Combination, ...callbacks: (((ct
   if (combination === 'overwrite') {
     // use last callback
     return async (ctx: T) => {
-      const callback = callbacks[callbacks.length - 1]
+      let callback
+      for (let i = callbacks.length - 1; i >= 0; i--) {
+        if (callbacks[i] != null) {
+          callback = callbacks[i]
+          break
+        }
+      }
       if (callback)
         return { ...ctx, ...(await callback(ctx)) }