redux-first-router/dist/reducer/createLocationReducer.js.flow

think of your app in states not routes (and, yes, while keeping the address bar in sync)

// @flow
import { NOT_FOUND, ADD_ROUTES } from '../index'
import isServer from '../pure-utils/isServer'
import { nestHistory } from '../pure-utils/nestAction'
import type {
  LocationState,
  RoutesMap,
  Action,
  Payload,
  History
} from '../flow-types'

export default (initialState: LocationState, routesMap: RoutesMap) => (
  state: LocationState = initialState,
  action: Action
): LocationState => {
  routesMap = state.routesMap || routesMap
  const route = routesMap[action.type]

  if (
    action.type === NOT_FOUND ||
    (route &&
      !action.error &&
      (typeof route === 'string' || route.path) &&
      (action.meta.location.current.pathname !== state.pathname ||
        action.meta.location.current.search !== state.search ||
        action.meta.location.kind === 'load'))
  ) {
    const query = action.meta.location.current.query
    const search = action.meta.location.current.search

    return {
      pathname: action.meta.location.current.pathname,
      type: action.type,
      payload: { ...action.payload },
      ...(query && { query, search }),
      prev: action.meta.location.prev,
      kind: action.meta.location.kind,
      history: action.meta.location.history,
      hasSSR: state.hasSSR,
      routesMap
    }
  }
  else if (
    route &&
      !action.error &&
      (typeof route === 'string' || route.path) &&
      (action.meta.location.current.pathname === state.pathname &&
        action.meta.location.current.search === state.search &&
        action.meta.location.kind !== state.kind
      )
  ) {
    return {
      ...state,
      kind: action.meta.location.kind
    }
  }
  else if (action.type === ADD_ROUTES) {
    return {
      ...state,
      routesMap: { ...state.routesMap, ...action.payload.routes }
    }
  }

  return state
}

export const getInitialState = (
  currentPathname: string,
  meta: ?{ search?: string, query?: Object },
  type: string,
  payload: Payload,
  routesMap: RoutesMap,
  history: History
): LocationState => ({
  search: currentPathname.split('?')[1],
  pathname: currentPathname.split('?')[0],
  type,
  payload,
  ...meta,
  prev: {
    pathname: '',
    type: '',
    payload: {}
  },
  kind: undefined,
  history: nestHistory(history),
  hasSSR: isServer() ? true : undefined, // client uses initial server `hasSSR` state setup here
  routesMap
})