History enabling redux reducer decorator

historyEnable.js

'use strict';

function historyEnable(reducer) {

    var initObject = {
        past: [],
        present: reducer(undefined, {}),
        future: []
    };

    // Return a reducer that handles back, forward, and setting history stops
    return function (state, action) {
        state = state || initObject;

        switch (action.type) {
            case 'GO_BACK':
                var previousState = state.past[state.past.length - 1];
                var newPast = state.past.slice(0, state.past.length - 1);
                var newFuture = [state.present].concat(state.future);
                return {
                    past: newPast,
                    present: previousState,
                    future: newFuture
                };
            case 'GO_FORWARD':
                return {
                    past: [].concat(state.past).concat([state.present]),
                    present: state.future[0],
                    future: state.future.slice(1)
                };
            case 'SET_HISTORY_POINT':
                return {
                    past: [].concat(state.past).concat([state.present]),
                    present: state.present,
                    future: []
                };
            default:
                // Handle an action without affecting history state
                return {
                    past: state.past,
                    present: reducer(state.present, action),
                    future: state.future
                };
        }
    };
}

module.exports = historyEnable;

historyEnable.test.js

'use strict';

var redux = require('redux');
var historyEnable = require('./historyEnable.js');

function reducer(state, action) {
    state = state === undefined ? 5 : state;
    switch (action.type) {
        case 'PLUS':
            return state + 1;
        case 'MINUS':
            return state - 1;
        default:
            return state;
    }
}

test('initialization of state should work', function() {

    var store = redux.createStore(
        historyEnable(reducer)
    );
    var state = store.getState();

    expect(state).toBeDefined();
    expect(state.past).toEqual([]);
    expect(state.future).toEqual([]);
    expect(state.present).toEqual(5);
});

test('setting history point should work', function() {
    var store = redux.createStore(
        historyEnable(reducer)
    );
    store.dispatch({
        type: 'SET_HISTORY_POINT'
    });
    var state = store.getState();
    expect(state.past).toEqual([5]);
    expect(state.future).toEqual([]);
    expect(state.present).toEqual(5);
});

test('navigating back should work', function() {
    var store = redux.createStore(
        historyEnable(reducer)
    );
    store.dispatch({
        type: 'SET_HISTORY_POINT'
    });
    store.dispatch({
        type: 'PLUS'
    });
    store.dispatch({
        type: 'SET_HISTORY_POINT'
    });
    store.dispatch({
        type: 'PLUS'
    });
    store.dispatch({
        type: 'GO_BACK'
    });
    var state = store.getState();
    expect(state.past).toEqual([5]);
    expect(state.future).toEqual([7]);
    expect(state.present).toEqual(6);
});

test('navigating forward should work', function() {
    var store = redux.createStore(
        historyEnable(reducer)
    );
    store.dispatch({
        type: 'SET_HISTORY_POINT'
    });
    store.dispatch({
        type: 'PLUS'
    });
    store.dispatch({
        type: 'SET_HISTORY_POINT'
    });
    store.dispatch({
        type: 'PLUS'
    });
    store.dispatch({
        type: 'GO_BACK'
    });
    store.dispatch({
        type: 'GO_FORWARD'
    });
    var state = store.getState();
    expect(state.past).toEqual([5, 6]);
    expect(state.future).toEqual([]);
    expect(state.present).toEqual(7);
});