mocking named exports with jest

math-utils.js

const ZERO = 0;
const ONE = 1;

export const add = (a, b) => a + b;
export const mul = (a, b) => a * b;

export const sum = (...args) => args.reduce(add, ZERO);
export const product = (...args) => args.reduce(mul, ONE);

test.js

import * as MathUtils from "../math-utils.js";

describe("MathUtils", () => {
    describe("sum", () => {
        /**
         * This test fails because the spy never gets called even
         * though the `add` function in math-utils.js is getting
         * called.
         */
        it("should call add", () => {
            // Arrange
            const addSpy = jest.spyOn(MathUtils, "add");
            
            // Act
            const result = MathUtils.sum(1,2,3);

            // Assert
            expect(result).toEqual(6);
            expect(addSpy).toHaveBeenCalled();
        });
    });
});