Can I mock one function exported from a typescript module that uses ES6 import style

Question

I have been struggling with some specific ES6 importing-style and using with Jest mocking in typescript.

It's very specific, so I wrote this example puzzle to best ask for help.

// animals.ts

export function getCanine() {
    return 'dog';
}

export function getFeline() {
    return 'cat';
}

// animals.test.ts

import { getCanine, getFeline } from './animals'; // you are not allowed to change this line
// Not changing the above line is _fundamental_ to my question.
// Lines may be added above or below it.

describe('Jest mocking tests', function () {

    it('output of getCanine() is successfully changed', async () => {

        expect(getCanine()).toEqual('dog');
        expect(getFeline()).toEqual('cat');

        // how do I mock getCanine() to return 'wolf' instead?

        expect(getCanine()).toEqual('wolf');
        expect(getFeline()).toEqual('cat');
    });
});

Answer 1

I think this would be a way to achieve that:

/* ... */
expect(getCanine()).toEqual('dog');
expect(getFeline()).toEqual('cat');

getCanine = jest.fn(getCanine).mockReturnValueOnce('wolf');

expect(getCanine()).toEqual('wolf');

mockFn.mockReturnValueOnce has been used so that the next time you're invoking getCanine(), it will fall back to the default implementation(i.e. returning 'dog').