redux/es/combineReducers.js

import { ActionTypes } from './createStore';
import isPlainObject from 'lodash-es/isPlainObject';
import warning from './utils/warning';

function getUndefinedStateErrorMessage(key, action) {
  var actionType = action && action.type;
  var actionName = actionType && '"' + actionType.toString() + '"' || 'an action';

  return 'Given action ' + actionName + ', reducer "' + key + '" returned undefined. ' + 'To ignore an action, you must explicitly return the previous state.';
}

function getUnexpectedStateShapeWarningMessage(inputState, reducers, action, unexpectedKeyCache) {
  var reducerKeys = Object.keys(reducers);
  var argumentName = action && action.type === ActionTypes.INIT ? 'preloadedState argument passed to createStore' : 'previous state received by the reducer';

  if (reducerKeys.length === 0) {
    return 'Store does not have a valid reducer. Make sure the argument passed ' + 'to combineReducers is an object whose values are reducers.';
  }

  if (!isPlainObject(inputState)) {
    return 'The ' + argumentName + ' has unexpected type of "' + {}.toString.call(inputState).match(/\s([a-z|A-Z]+)/)[1] + '". Expected argument to be an object with the following ' + ('keys: "' + reducerKeys.join('", "') + '"');
  }

  var unexpectedKeys = Object.keys(inputState).filter(function (key) {
    return !reducers.hasOwnProperty(key) && !unexpectedKeyCache[key];
  });

  unexpectedKeys.forEach(function (key) {
    unexpectedKeyCache[key] = true;
  });

  if (unexpectedKeys.length > 0) {
    return 'Unexpected ' + (unexpectedKeys.length > 1 ? 'keys' : 'key') + ' ' + ('"' + unexpectedKeys.join('", "') + '" found in ' + argumentName + '. ') + 'Expected to find one of the known reducer keys instead: ' + ('"' + reducerKeys.join('", "') + '". Unexpected keys will be ignored.');
  }
}

function assertReducerSanity(reducers) {
  Object.keys(reducers).forEach(function (key) {
    var reducer = reducers[key];
    var initialState = reducer(undefined, { type: ActionTypes.INIT });

    if (typeof initialState === 'undefined') {
      throw new Error('Reducer "' + key + '" returned undefined during initialization. ' + 'If the state passed to the reducer is undefined, you must ' + 'explicitly return the initial state. The initial state may ' + 'not be undefined.');
    }

    var type = '@@redux/PROBE_UNKNOWN_ACTION_' + Math.random().toString(36).substring(7).split('').join('.');
    if (typeof reducer(undefined, { type: type }) === 'undefined') {
      throw new Error('Reducer "' + key + '" returned undefined when probed with a random type. ' + ('Don\'t try to handle ' + ActionTypes.INIT + ' or other actions in "redux/*" ') + 'namespace. They are considered private. Instead, you must return the ' + 'current state for any unknown actions, unless it is undefined, ' + 'in which case you must return the initial state, regardless of the ' + 'action type. The initial state may not be undefined.');
    }
  });
}

/**
 * Turns an object whose values are different reducer functions, into a single
 * reducer function. It will call every child reducer, and gather their results
 * into a single state object, whose keys correspond to the keys of the passed
 * reducer functions.
 *
 * @param {Object} reducers An object whose values correspond to different
 * reducer functions that need to be combined into one. One handy way to obtain
 * it is to use ES6 `import * as reducers` syntax. The reducers may never return
 * undefined for any action. Instead, they should return their initial state
 * if the state passed to them was undefined, and the current state for any
 * unrecognized action.
 *
 * @returns {Function} A reducer function that invokes every reducer inside the
 * passed object, and builds a state object with the same shape.
 */
export default function combineReducers(reducers) {
  var reducerKeys = Object.keys(reducers);
  var finalReducers = {};
  for (var i = 0; i < reducerKeys.length; i++) {
    var key = reducerKeys[i];

    if (process.env.NODE_ENV !== 'production') {
      if (typeof reducers[key] === 'undefined') {
        warning('No reducer provided for key "' + key + '"');
      }
    }

    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key];
    }
  }
  var finalReducerKeys = Object.keys(finalReducers);

  if (process.env.NODE_ENV !== 'production') {
    var unexpectedKeyCache = {};
  }

  var sanityError;
  try {
    assertReducerSanity(finalReducers);
  } catch (e) {
    sanityError = e;
  }

  return function combination() {
    var state = arguments.length <= 0 || arguments[0] === undefined ? {} : arguments[0];
    var action = arguments[1];

    if (sanityError) {
      throw sanityError;
    }

    if (process.env.NODE_ENV !== 'production') {
      var warningMessage = getUnexpectedStateShapeWarningMessage(state, finalReducers, action, unexpectedKeyCache);
      if (warningMessage) {
        warning(warningMessage);
      }
    }

    var hasChanged = false;
    var nextState = {};
    for (var i = 0; i < finalReducerKeys.length; i++) {
      var key = finalReducerKeys[i];
      var reducer = finalReducers[key];
      var previousStateForKey = state[key];
      var nextStateForKey = reducer(previousStateForKey, action);
      if (typeof nextStateForKey === 'undefined') {
        var errorMessage = getUndefinedStateErrorMessage(key, action);
        throw new Error(errorMessage);
      }
      nextState[key] = nextStateForKey;
      hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
    }
    return hasChanged ? nextState : state;
  };
}