Mock doesn't work well

[PH8919]

Hello,
I am new to TS and finished some code. Now i want to prepare UT for the codes. but I encounter one issues. For below sample code,
sum.ts:

export function add(x, y) {
return add1(x, y);
}
export function add1(x, y) {
return x + y;
}

I want to write the UT for add function in sum.ts. at this time, i try to mock the add1 function. But i failed, below is my test code.
sum.test.ts:

import * as sum from '../src/sum';
test('adds 1 + 2 to equal 3', () => {
jest.spyOn(sum, 'add1').mockImplementation((x, y) => 10);
const result = sum.add(1, 2);
expect(result).toBe(10);
});

Though i use spyOn to mock the "add1", it doesn't work. the result is as follows:

FAIL tests/cli.test.ts (15.182 s)
× adds 1 + 2 to equal 3 (5 ms)
● adds 1 + 2 to equal 3
expect(received).toBe(expected) // Object.is equality
Expected: 10
Received: 3

how can I mock the "add1" to make the UT for "add" function?
Can anyone help me?

Thanks!

[PH8919]

I also tried below test code and it doesn't work.

jest.mock('../src/sum', () => {
const original = jest.requireActual('../src/sum');
return {
__esModule: true,
...original,
add1: jest.fn((x, y) => x - y),
};
});
import * as sum from '../src/sum';
it('should use mocked add1', () => {
expect(sum.add(5, 3)).toBe(2);
});