I'm importing Enter code here...
//Utils file
export default = () => {}
//Other file
import * as Utils from 'lib/utils';
export const foo = () => Utils.default();
//test file
import * as Utils from 'lib/utils';
import * as Foo from 'lib/foo';
it('should call default from utils file', ()=> {
const myStub = sinon.stub(Utils, 'default');
Foo.foo();
})