How to mock react custom hook returned value?

Load the hook as a module. Then mock the module:

jest.mock('module_name', () => ({
    useClientRect: () => [300, 200, jest.fn()]
}));

mock should be called on top of the file outside test fn. Therefore we are going to have only one array as the mocked value.

If you want to mock the hook with different values in different tests:

import * as hooks from 'module_name';

it('a test', () => {
    jest.spyOn(hooks, 'useClientRect').mockImplementation(() => ([100, 200, jest.fn()]));
    //rest of the test
});