For returning new state in redux with nested objects from a reducer's initialState

Redux nested Object.assign()

let state = {
  nestedObj: {
    x: 1,
    y: 2,
  },
  h: 10
  
}

let addObj = {
  z: 3
}

fn = () => {
  return Object.assign( {}, state, { nestedObj : Object.assign( {}, state.nestedObj, addObj ) } );
}

fn();

A better way to handle this however is to separate the parts of state that you want to renew in different reducers. Then make state default to that specific portion of state with ( state = intialState.nestedObj , action )