<link rel="import" href="bower_components/polymer-redux/polymer-redux.html">

<script>

    // Default state
    const initialState = {
        comments: [
            {text: 'Hello Polymer 🦄', id: 0},
            {text: 'Hello Redux 😎', id: 1}
        ]
    }

    // Reducer
    const reducer = (state, action) => {

        // Set deafult state
        if (!state) return initialState;

        // Actions
        switch (action.type) {

            // ADD_COMMENT
            // ------------------------
            case 'ADD_COMMENT':

            // add new array item
            const commentsList = state.comments.slice(0);
            const initialId = commentsList.length ? commentsList[commentsList.length - 1].id + 1 : 0
            commentsList.push({
                text: action.comment, 
                id: initialId
            });

            // set new state
            return Object.assign({}, state, { comments: commentsList });
            break;

            // DELETE_COMMENT
            // ------------------------
            case 'DELETE_COMMENT':

            // remove item from array
            const newList = state.comments.filter((c) => {
                return c.id !== action.id
            });

            // set new state
            return Object.assign({}, state, { comments: newList });
            break;

        }

    }

    // Store
    const store = Redux.createStore(reducer);

    // Create behavior for polymer elements
    const ReduxBehavior = PolymerRedux(store);

</script>

<link rel="import" href="bower_components/polymer-redux/polymer-redux.html">

<script>

    // Default state
    const initialState = {
        comments: [
            {text: 'Hello Polymer 🦄', id: 0},
            {text: 'Hello Redux 😎', id: 1}
        ]
    }

    // Reducer
    const reducer = (state, action) => {

        // Set deafult state
        if (!state) return initialState;

        // Actions
        switch (action.type) {

            // ADD_COMMENT
            // ------------------------
            case 'ADD_COMMENT':

            // add new array item
            const commentsList = state.comments.slice(0);
            const generateId = commentsList.length ? commentsList[commentsList.length - 1].id + 1 : 0
            commentsList.push({
                text: action.comment, 
                id: generateId
            });

            // set new state
            return Object.assign({}, state, { comments: commentsList });
            break;

            // DELETE_COMMENT
            // ------------------------
            case 'DELETE_COMMENT':

            // remove item from array
            const newList = state.comments.filter((c) => {
                return c.id !== action.id
            });

            // set new state
            return Object.assign({}, state, { comments: newList });
            break;

        }

    }

    // Store
    const store = Redux.createStore(reducer);

    // Create behavior for polymer elements
    const ReduxBehavior = PolymerRedux(store);

</script>

Change variable name to more readable
Change variable name to more readable