How to add ID and class in nav menu walker?

I would like to set up a custom walker that produces the following output:

 <div id="navbarHolder">
    <div id="navbar">
        <ul>
            <li><a id="nav_about" href="#about">ABOUT<span class="m_w">ABOUT</span><span class="m_r">ABOUT</span></a></li>
            <li><a id="nav_work" href="#work">WORK<span class="m_w">WORK</span><span class="m_r">WORK</span></a></li>
            <li><a id="nav_projects" href="#projects">Projects<span class="m_w">Projects</span><span class="m_r">Projects</span></a></li>
            <li><a id="nav_contact" href="#contact">Contact<span class="m_w">Contact</span><span class="m_r">Contact</span></a></li>
        </ul>
    </div>
</div><!-- end div #navbarHolder -->

My current walker code is as follows:

<?php
class MFC_Walker_Nav_Menu extends Walker_Nav_Menu {

    /**
     * @see Walker::start_el()
     * @since 3.0.0
     *
     * @param string $output Passed by reference. Used to append additional content.
     * @param object $item Menu item data object.
     * @param int $depth Depth of menu item. Used for padding.
     * @param int $current_page Menu item ID.
     * @param object $args
     */
    function start_el(&$output, $item, $depth, $args) {
        global $wp_query;           

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value="";

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
        $class_names=" class="" . esc_attr( $class_names ) . '"';

        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
        $id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

        $output .= $indent . '<li' . $id . $value . $class_names .'>';

        $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
        $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
        $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

                // new addition for active class on the a tag
                if(in_array('current-menu-item', $classes)) {
                    $attributes .= ' class="active"';
                }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
}