astx-redux-util/lib/reducer/joinReducers.js

'use strict';

exports.__esModule = true;
exports.default = joinReducers;

var _lodash = require('lodash.isfunction');

var _lodash2 = _interopRequireDefault(_lodash);

var _lodash3 = require('lodash.last');

var _lodash4 = _interopRequireDefault(_lodash3);

var _verify = require('../util/verify');

var _verify2 = _interopRequireDefault(_verify);

function _interopRequireDefault(obj) { return obj && obj.__esModule ? obj : { default: obj }; }

/**
 * Create a higher-order reducer by combining two or more reducers,
 * logically executing each in sequence (in essence combining their
 * functionality into one).  This is useful when combining various
 * reducer types into one logical construct.
 * 
 * **Please Note:** Because each reducer is able to build on what has
 * been accomplished by a prior reducer, joinReducers cumulatively
 * passes the state parameter that was returned from any prior reducer
 * (in the chain of reducers to execute).  In essence this is an
 * accumulative process.  While this does NOT relax the immutable
 * constraint of the reducer's state parameter, it is possible for a
 * down-stream reducer to receive a state parameter that is a
 * different instance from the start of the reduction process (because
 * an up-stream reducer needed to alter it in some way).
 * 
 * The {{book.guide.devGuide}} discusses joinReducers() in more detail
 * (see {{book.guide.conceptJoin}}), and additional examples can
 * be found in {{book.guide.fullExample}}.
 *
 * @param {...reducerFn} reducerFns two or more reducer functions to join
 * together.
 *
 * @param {InitialState} [initialState] - the optional fall-back state
 * value used during the state initialization boot-strap process.
 * 
 * @returns {reducerFn} a newly created reducer function (described above).
 */
function joinReducers() {
  for (var _len = arguments.length, reducerFns = Array(_len), _key = 0; _key < _len; _key++) {
    reducerFns[_key] = arguments[_key];
  }

  // define our initialState parameter (optionally, the last parameter)
  // NOTE: We have to do this programatically because our function 
  //       accepts variable number of arguments.
  var initialState = (0, _lodash2.default)((0, _lodash4.default)(reducerFns)) ? undefined : reducerFns.pop();

  // validate params
  var check = _verify2.default.prefix('AstxReduxUtil.joinReducers() parameter violation: ');

  check(reducerFns && reducerFns.length >= 2, 'two or more reducerFn arguments are required');

  // ... each arg MUST be a function (reducerFn)
  var badArgNum = reducerFns.reduce(function (firstBadArgNum, reducerFn, indx) {
    return firstBadArgNum || ((0, _lodash2.default)(reducerFn) ? 0 : indx + 1);
  }, 0);
  check(!badArgNum, 'argument position number ' + badArgNum + ' is NOT a function ... expecting two or more reducerFns to join together');

  // expose our new higher-order reducer
  return function () {
    var state = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : initialState;
    var action = arguments[1];
    var originalReducerState = arguments[2];


    // maintain the originalReducerState as the immutable state
    // at the time of the start of the reduction process
    // ... in support of joinReducers()
    // ... for more info, refer to the Dev Guide {{book.guide.originalReducerState}}
    if (originalReducerState === undefined) {
      originalReducerState = state;
    }

    // execute each reducerFn in sequence
    return reducerFns.reduce(function (nextState, reducerFn) {
      return reducerFn(nextState, action, originalReducerState);
    }, state);
  };
}