Example to mock an exported function

my-function.spec.ts

import * as functionToMock from '../src/other-function';
import { myFunction } from '../src/my-function';
import { stub } from 'sinon';

describe('myFunction()', () => {
  beforeEach(() => {
    stub(functionToMock, 'otherFunction').returns('expected result' as any);
  });

  it('should return otherFunction result', () => {
    const result = myFunction();

    expect(functionToMock.otherFunction).to.have.been.calledOnceExactlyWith(123);
    expect(result).to.be.eq('expected result');
  });
});