From df77d8cd8bbee4d933797e00eb0cee452c25d1f9 Mon Sep 17 00:00:00 2001
From: Wonsuk Choi <sukvvon@gmail.com>
Date: Wed, 15 Apr 2026 16:47:54 +0900
Subject: [PATCH] test(svelte-query/createMutation): add type tests for generic
 type inference and callbacks (#10502)

---
 .../createMutation/createMutation.test-d.ts   | 138 ++++++++++++++++++
 1 file changed, 138 insertions(+)
 create mode 100644 packages/svelte-query/tests/createMutation/createMutation.test-d.ts

diff --git a/packages/svelte-query/tests/createMutation/createMutation.test-d.ts b/packages/svelte-query/tests/createMutation/createMutation.test-d.ts
new file mode 100644
index 00000000000..4ea6027949e
--- /dev/null
+++ b/packages/svelte-query/tests/createMutation/createMutation.test-d.ts
@@ -0,0 +1,138 @@
+import { describe, expectTypeOf, it } from 'vitest'
+import { QueryClient } from '@tanstack/query-core'
+import { createMutation } from '../../src/index.js'
+import type { DefaultError } from '@tanstack/query-core'
+import type { CreateMutationResult } from '../../src/types.js'
+
+describe('createMutation', () => {
+  it('should infer TData from mutationFn return type', () => {
+    const mutation = createMutation(() => ({
+      mutationFn: () => Promise.resolve('data'),
+    }))
+
+    expectTypeOf(mutation.data).toEqualTypeOf<string | undefined>()
+    expectTypeOf(mutation.error).toEqualTypeOf<DefaultError | null>()
+  })
+
+  it('should infer TVariables from mutationFn parameter', () => {
+    const mutation = createMutation(() => ({
+      mutationFn: (vars: { id: string }) => Promise.resolve(vars.id),
+    }))
+
+    expectTypeOf(mutation.mutate).toBeCallableWith({ id: '1' })
+    expectTypeOf(mutation.data).toEqualTypeOf<string | undefined>()
+  })
+
+  it('should infer TOnMutateResult from onMutate return type', () => {
+    createMutation(() => ({
+      mutationFn: () => Promise.resolve('data'),
+      onMutate: () => {
+        return { token: 'abc' }
+      },
+      onSuccess: (_data, _variables, onMutateResult) => {
+        expectTypeOf(onMutateResult).toEqualTypeOf<{ token: string }>()
+      },
+      onError: (_error, _variables, onMutateResult) => {
+        expectTypeOf(onMutateResult).toEqualTypeOf<
+          { token: string } | undefined
+        >()
+      },
+    }))
+  })
+
+  it('should allow explicit generic types', () => {
+    const mutation = createMutation<string, Error, { id: number }>(() => ({
+      mutationFn: (vars) => {
+        expectTypeOf(vars).toEqualTypeOf<{ id: number }>()
+        return Promise.resolve('result')
+      },
+    }))
+
+    expectTypeOf(mutation.data).toEqualTypeOf<string | undefined>()
+    expectTypeOf(mutation.error).toEqualTypeOf<Error | null>()
+  })
+
+  it('should return correct CreateMutationResult type', () => {
+    const mutation = createMutation(() => ({
+      mutationFn: () => Promise.resolve(42),
+    }))
+
+    expectTypeOf(mutation).toEqualTypeOf<
+      CreateMutationResult<number, DefaultError, void, unknown>
+    >()
+  })
+
+  it('should type mutateAsync with correct return type', () => {
+    const mutation = createMutation(() => ({
+      mutationFn: (id: string) => Promise.resolve(id.length),
+    }))
+
+    expectTypeOf(mutation.mutateAsync).toBeCallableWith('test')
+    expectTypeOf(mutation.mutateAsync('test')).toEqualTypeOf<Promise<number>>()
+  })
+
+  it('should default TVariables to void when mutationFn has no parameters', () => {
+    const mutation = createMutation(() => ({
+      mutationFn: () => Promise.resolve('data'),
+    }))
+
+    expectTypeOf(mutation.mutate).toBeCallableWith()
+  })
+
+  it('should infer custom TError type', () => {
+    class CustomError extends Error {
+      code: number
+      constructor(code: number) {
+        super()
+        this.code = code
+      }
+    }
+
+    const mutation = createMutation<string, CustomError>(() => ({
+      mutationFn: () => Promise.resolve('data'),
+    }))
+
+    expectTypeOf(mutation.error).toEqualTypeOf<CustomError | null>()
+    expectTypeOf(mutation.data).toEqualTypeOf<string | undefined>()
+  })
+
+  it('should infer types for onSettled callback', () => {
+    createMutation(() => ({
+      mutationFn: () => Promise.resolve(42),
+      onSettled: (data, error, _variables, _onMutateResult) => {
+        expectTypeOf(data).toEqualTypeOf<number | undefined>()
+        expectTypeOf(error).toEqualTypeOf<DefaultError | null>()
+      },
+    }))
+  })
+
+  it('should infer custom TError in onError callback', () => {
+    class CustomError extends Error {
+      code: number
+      constructor(code: number) {
+        super()
+        this.code = code
+      }
+    }
+
+    createMutation<string, CustomError>(() => ({
+      mutationFn: () => Promise.resolve('data'),
+      onError: (error) => {
+        expectTypeOf(error).toEqualTypeOf<CustomError>()
+      },
+    }))
+  })
+
+  it('should accept queryClient as second argument', () => {
+    const queryClient = new QueryClient()
+
+    const mutation = createMutation(
+      () => ({
+        mutationFn: () => Promise.resolve('data'),
+      }),
+      () => queryClient,
+    )
+
+    expectTypeOf(mutation.data).toEqualTypeOf<string | undefined>()
+  })
+})