Selector dependency injection

selector-dep-injection.js

// It gets hard to test combined selectors after a while.
// The dependency chains run deep.  To get around this we
// can test the callback function seperately from the rest
// of the selector, but that leaves a testing gap.

// In order to test that createSelector gets called with the
// right stuff, we could do something like this

// WARNING UNTESTED CODE

// In selector
import { selectedFooIds, foosHash } from './selectors';
import { lookUpByKey } from './transformers';

export const selectorHash = {
  getSelectedFoos: () => createSelector(selectedFooIds, foosHash, lookUpByKey)
};

export default build(selectorHash);

// Generally utility to call each member of selecorHash
function build(hash) {
  return Object.keys(hash)
    .reduce(function(output, key) {
      output[key] = hash[key]();
      return output;
    }, {});
}

/////

// In test
// Inside your first describe
let selectors;
let createSelectorSpy;
beforeEach(() => {
  createSelectorSpy = sinon.stub(reselect.createSelector);
  selectors = selectorHash.getSelectedFoos()
});

it('calls createSelector with correct values', () => {
  expect(createSelectorSpy)
    .to.have.been.calledWith(selectedFooIds, foosHash, lookUpByKey);
});
// now we can test what createSelectorSpy gets called with