Redux undo reducer factory example by @iclanzan

ReduxUndoExample.js

// Written by @iclanzan
// All credit goes to him!

// You create the reducer like this:
// var reducer = createTimelineReducer('someName', someReducer, ['foo', 'bar']);
// And then whenever an action of type `foo` or `bar` happens it calls `someReducer` and adds the result to the timeline.
// Then to undo/redo you trigger an action of type `someNameUndo`/`someNameRedo`.

var defaults = require('lodash/object/defaults');
var capitalize = require('lodash/string/capitalize');

var timelineActionTypes = ['add', 'seek', 'undo', 'redo'];

exports = module.exports = function (name, reducer, actionTypes, size) {
  var initialState = {
    timeline: [],
    index: -1,
    size: size || 1024
  };

  var handlers = timelineActionTypes.reduce(function (result, type) {
    result[name + capitalize(type)] = exports[type];
    return result;
  }, {});

  return function (state, action) {
    if (state === undefined) {
      state = initialState;
    }

    var actionType = action.type;
    var handler = handlers[actionType];

    if (handler) {
      return handler(state, action.payload);
    }

    if (!~actionTypes.indexOf(actionType)) {
      return state;
    }

    return add(state, reducer(state.current, action));
  };
};

var add = exports.add = function (state, data) {
  var current = state.current;

  if (current == data) {
    return state;
  }

  var nextState = {current: data};
  var nextTimeline = nextState.timeline = state.timeline.slice(
    state.index + 1 >= state.size ? 1 : 0,
    state.index + 1
  );

  nextState.index = nextTimeline.push(data) - 1;

  return defaults(nextState, state);
};

var seek = exports.seek = function (state, index) {
  var timeline = state.timeline;
  var maxIndex = timeline.length - 1;

  if (index < 0) {
    index = 0;
  }

  // Allow for -1 when timeline is empty.
  if (index > maxIndex) {
    index = maxIndex;
  }

  return index == state.index ? state : defaults({
    index: index,
    current: timeline[index]
  }, state);
};

exports.undo = function (state, steps) {
  return seek(state, state.index - (steps || 1));
};

exports.redo = function (state, steps) {
  return seek(state, state.index + (steps || 1));
};