How to mock function using jest.fn() - jestjs

Question

The jest document says, There are two ways to get your hands on mock functions: Either by require()ing a mocked component (via jest.mock('moduleName')) or by explicitly requesting one from jest.fn() in your test:
So I am trying mocking a function thinking whenever a test makes a call to the minus function, it executes the mocked function and returns 10 instead of a*b but it's not working the way I thought.
app.js:
function add(a, b) {
return multiply(a, b) + b;
}
function multiply(a, b) {
return a * b;
}
module.exports = add;
add-test.js
const add = require('./add');
describe('add', () => {
it('should add two numbers', () => {
const multiply = jest.fn((a, b) => {10})
expect(add(1, 2)).toBe(12);
});
});
Note: If I change expect(add(1, 2)).toBe(12); to expect(add(1, 2)).toBe(4); the test will pass, but I want to mock multiply and return 10. So when I actually call made to multiply function then it should invoke mocked function and not real implementation.

Answer 1

You will have to change the structure of app.js slightly to test this the way you want.
You can either extract multiply() into its own file like this:
mulitiply.js
function multiply(a, b) {
return a * b;
}
app.js
function add(a, b) {
return multiply(a, b) + b;
}
add.test.js
const add = require('./add');
const multiply = require('./multiply');
jest.mock('multiply', () => jest.fn(a, b) => 10)
describe('add', () => {
it('adds the numbers', () => {
expect(add(1, 2)).toBe(12);
});
});
or change the implementation to be something like this:
add.js
function add(multiply) {
multiply()
}
add.test.js
const add = require('./add');
describe('add', () => {
it('adds the numbers', () => {
const multiply = jest.fn(a,b) => 10;
expect(add(multiply(1, 2)).toBe(12);
});
});