Branch Coverage for Redux reducer

Question

Say I have the following reducer:

import {FOO} from '../const/Foo'

const myReducer = (initialState = {foo: ''}, action) => {
  const state = {}
  if (action) {
    switch (action.type) {
      case FOO:
        state.foo = action.foo
    };
  }
  return Object.assign({}, initialState, state)
}

and I test using jest with:

import FOO from '../const/Foo'

test('returns correct state when action is not "Foo"', () => {
  expect(myReducer({foo: 'bar'}, {type: 'foo'})).toEqual({foo: 'bar'})
})

test("returns correct state when action is 'Foo'", () => {
  expect(myReducer({}, {type: FOO, foo: 'bar'})).toEqual({foo: 'bar'})
})

test('when there is no action / testing the default', () => {
  expect(myReducer()).toEqual({foo: ''})
})

This produces a branch coverage of 4/5. After a little bit of thinking/removing and/or re-adding lines I've arrived at the branching logic being on the initialState set. Which almost makes sense. Except:

1) Why doesn't the last test, with an empty myReducer() call cover this case.

When the reducer is pared down to:

const myReducer = (initialState = {foo: ''}, action) => {
    const state = {}
    return Object.assign({}, initialState, state)
}

The tests (which now fail) have a branch coverage of 1/1.

What is going on here?

EDIT: As per request, I am adding the configuration. I pass the following jest.json to jest:

{

  "bail": true,
  "verbose": true,
  "moduleNameMapper": {
    "\\.(sass|jpg|png)$": "<rootDir>/src/main/js/config/emptyExport.js"
  },
  "testRegex": ".*(?<!snapshot)\\.(test|spec)\\.js$",
  "collectCoverage": true,
  "collectCoverageFrom": ["src/main/js/**/*.js"
    , "!**/node_modules/**"
    , "!**/*spec.js"
    , "!src/main/js/config/emptyExport.js"
    , "!**/coverage/**/*.js"
    , "!src/main/js/app.js"
    , "!src/main/js/store/configureStore.js"
    , "!src/main/js/reducers/index.js"],
  "coverageDirectory": "<rootDir>/src/main/js/coverage",
  "coverageThreshold": {
    "global": {
      "branches": 85,
      "function": 95,
      "lines": 95,
      "statements": 95
    }
  }
}

EDIT2: The following test also does not affect test coverage:

test('when there is no action / testing the default', () => {
  expect(addressReducer(undefined, {foo: 'bar'})).toEqual({address: ''})
})

I still do not understand why the initial default test implementation was not equivalent from a branch coverage perspective.

Answer 1

the last test with myReducer() covers the case where the default value for initialState which is in your case {foo: ''} gets applied.