How do I mock a promise in reactjs?

I want to write a test which mocks a promise in reactjs

I just need a mocked implementation of getHeaders() to return a string

export const loadAllProjects = () => {
    return (dispatch) => {
        getHeaders()
            .then(headers => {
                ...do stuff
            })
    }
}

to clarify my original function was...

export const loadAllProjects = () => {
    return (dispatch) => {
                ...do stuff
    }
}

...and my test was...

    it('should create SET_ALL_PROJECTS action when fetching projects', () => {
    fetchMock
        .getOnce('http://test.projects.api/api/projects',
        {
            body: [{name: "x"}],
            headers: { 'content-type': 'application/json' }
        }).spy()
    const expectedActions = [
        { type: "SET_ALL_PROJECTS", json: [{name:"x"}] },
    ]
    checkAsyncActionsWereDispatched(expectedActions, actions.loadAllProjects)
});

I want the test to work with the mocked header

const getHeaders = () => {
    return new Promise((resolve, reject) => {
        resolve("some string");
    });
};

a = await getHeaders();    //some string

return Promise.resolve("your headers here");

const mockPostSpy = jest
.spyOn(axios, 'post')
.mockImplementation(() => {
    return new Promise((resolve) => {
        return resolve({
            data: {},
        });
    });
});

const mockPostSpy = jest
.spyOn(axios, 'post')
.mockImplementation(() => {
    return new Promise((resolve) => {
        return reject({});
    });
});