handy-methods-test.js

import handyMethods from 'handy-methods';

describe('handyMethods', function() {
  describe('methodB', function() {
    it("delegates to methodA", function() {
      let methodAStub = sinon.stub(handyMethods, 'methodB');
      methodAStub.returns('not foo');
      expect(methodB('bar')).to.equal('not foo');
    })
  })
});

handy-methods.js

var methodA = function(arg) {
  // complex logic here
  return 'foo';
};

var methodB = function(arg) {
  // complex logic here
  return methodA(arg);
};

export {
  methodA,
  methodB
};

The call to methodA in methodB is always the closure value of methodA. In node module land I could restub that value using rewire. I'm not sure how to do it in ES2015 babel land.