Edit in JSFiddle

var AppState = function (initialState) {
    this.data = initialState;
    this.callbacks = [];
};

AppState.prototype.get = function () {
    return this.data;
};

AppState.prototype.set = function (newState) {
    var merged = _.extend(this.data, newState);
    this.data = merged;
    this.callbacks.forEach(function (cb) {
        cb.call(null, merged);
    });
};

AppState.prototype.addListener = function (cb) {
    this.callbacks.push(cb);
};

var PostsList = function (container, data) {
    this.container = container;
    this.setData(data);
};

PostsList.prototype.render = function () {
    var ul = document.createElement('ul');
    var lis = this.data.map(function (post) {
        var li = document.createElement('li');
        li.innerText = post.title;
        ul.appendChild(li);
    });
    while (this.container.hasChildNodes()) {
        this.container.removeChild(this.container.lastChild);
    }
    this.container.appendChild(ul);
};

PostsList.prototype.setData = function (newData) {
    this.data = newData;
    this.render();
};

var Actions = {
    addPost: function (newPost) {
        var currentPosts = appState.get().posts;
        currentPosts.push(newPost);
        appState.set({
            posts: currentPosts
        });
    }
};

var initialDataset = [{
    title: 'My new blog post'
}, {
    title: 'Managing state'
}];

var appState = new AppState({
    posts: initialDataset
});
var postsList = new PostsList(document.getElementById('main'), appState.get().posts);

appState.addListener(function (newData) {
    postsList.setData(newData.posts);
});

setInterval(function () {
    var counter = appState.get().posts.length + 1;
    Actions.addPost({
        title: 'Post n. ' + counter
    });
}, 1000);

<div id="main"></div>