state.js

{
  users: [{ id: "1231", username: "Dale" }, { id: "1235", username: "Sarah"}]
}

{
  type: "ADD_USER",
  payload: { user: { id: "5123", username: "Kyle" } }
}

const userReducer = (state, action) => {
    switch (action.type) {
       case "ADD_USER":
          return { ...state, users: [...state.users, action.payload.user ]}
       default:
          return state;
    }
}