Merge pull request #32864 from jerico-dev/fix-jest-fn

[jest] fix jest.fn to match spec

types/jest-in-case/jest-in-case-tests.ts

@@ -13,8 +13,8 @@ function subtract(minuend: number, subtrahend: number) {
 beforeEach(() => {
     jest.spyOn(global, 'describe').mockImplementation((title, fn) => (fn as () => void)());
     jest.spyOn(global, 'test').mockImplementation((name, fn) => (fn as () => void)());
-    global.test.skip = jest.fn((name, fn) => fn());
-    global.test.only = jest.fn((name, fn) => fn());
+    global.test.skip = jest.fn((_: string, fn: jest.EmptyFunction) => fn());
+    global.test.only = jest.fn((_: string, fn: jest.EmptyFunction) => fn());
 });
 
 afterEach(() => {

types/jest/index.d.ts

@@ -121,7 +121,7 @@ declare namespace jest {
     /**
      * Creates a mock function. Optionally takes a mock implementation.
      */
-    function fn<T, Y extends any[]>(implementation: (...args: Y) => T): Mock<T, Y>;
+    function fn<T, Y extends any[]>(implementation?: (...args: Y) => T): Mock<T, Y>;
     /**
      * Use the automatic mocking system to generate a mocked version of the given module.
      */

types/jest/jest-tests.ts

@@ -308,10 +308,15 @@ const mock7 = jest.fn((arg: number) => arg);
 const mock8: jest.Mock = jest.fn((arg: number) => arg);
 // $ExpectType Mock<Promise<boolean>, [number, string, {}, [], boolean]>
 const mock9 = jest.fn((a: number, _b: string, _c: {}, _iReallyDontCare: [], _makeItStop: boolean) => Promise.resolve(_makeItStop));
-// $ExpectType Mock<never, [never]>
-const mock10 = jest.fn((arg: never) => { throw new Error(arg); });
+// $ExpectType Mock<never, []>
+const mock10 = jest.fn(() => { throw new Error(); });
 // $ExpectType Mock<unknown, [unknown]>
 const mock11 = jest.fn((arg: unknown) => arg);
+interface TestApi {
+    test(x: number): string;
+}
+// $ExpectType Mock<string, [number]>
+const mock12 = jest.fn<ReturnType<TestApi["test"]>, ArgsType<TestApi["test"]>>();
 
 // $ExpectType number
 mock1('test');