Been playing with this for hours but not getting anywhere, please help!

So I have a menu which works fine for the sub menu dropdown on desktop.

What I want for mobile menus is for their to be an icon, next to all li that have .menu-item-has-children. Then when the icon is clicked it changes to adds a class of 'active' to the sub-menu. When click again, the active class to be removed and the icon to return back to

The html is

<ul id="primary-menu" class="menu nav-menu">
<li class="menu-item "><a href="#">Page One</a></li>
<li class="menu-item menu-item-has-children"><a href="#">Page Two</a>
<ul class="sub-menu">
    <li class="menu-item"><a href="#">Page Five</a></li>
    <li class="menu-item"><a href="#">Page Three</a></li>
    <li class="menu-item"><a href="#">Page Six</a></li>
</ul>
</li>
<li class="menu-item menu-item-has-children"><a href="#">Page Four</a>
<ul class="sub-menu">
    <li class="menu-item"><a href="#">Page Seven</a></li>
</ul>
</li>
<li class="menu-item"><a href="#">Page Eight</a></li>
</ul>

The JS for it so far is

( function() {
    var container, button, menu, links, i, len;

    container = document.getElementById( 'site-navigation' );
    if ( ! container ) {
        return;
    }

    button = container.getElementsByTagName( 'button' )[0];
    if ( 'undefined' === typeof button ) {
        return;
    }

    menu = container.getElementsByTagName( 'ul' )[0];

    // Hide menu toggle button if menu is empty and return early.
    if ( 'undefined' === typeof menu ) {
        button.style.display = 'none';
        return;
    }

    if ( -1 === menu.className.indexOf( 'nav-menu' ) ) {
        menu.className += ' nav-menu';
    }

    button.onclick = function() {
        if ( -1 !== container.className.indexOf( 'toggled' ) ) {
            container.className = container.className.replace( ' toggled', '' );
            button.setAttribute( 'aria-expanded', 'false' );
        } else {
            container.className += ' toggled';
            button.setAttribute( 'aria-expanded', 'true' );
        }
    };

    // Close small menu when user clicks outside
    document.addEventListener( 'click', function( event ) {
        var isClickInside = container.contains( event.target );

        if ( ! isClickInside ) {
            container.className = container.className.replace( ' toggled', '' );
            button.setAttribute( 'aria-expanded', 'false' );
        }
    } );

    // Get all the link elements within the menu.
    links = menu.getElementsByTagName( 'a' );

    // Each time a menu link is focused or blurred, toggle focus.
    for ( i = 0, len = links.length; i < len; i++ ) {
        links[i].addEventListener( 'focus', toggleFocus, true );
        links[i].addEventListener( 'blur', toggleFocus, true );
    }

    /**
     * Sets or removes .focus class on an element.
     */
    function toggleFocus() {
        var self = this;

        // Move up through the ancestors of the current link until we hit .nav-menu.
        while ( -1 === self.className.indexOf( 'nav-menu' ) ) {
            // On li elements toggle the class .focus.
            if ( 'li' === self.tagName.toLowerCase() ) {
                if ( -1 !== self.className.indexOf( 'focus' ) ) {
                    self.className = self.className.replace( ' focus', '' );
                } else {
                    self.className += ' focus';
                }
            }

            self = self.parentElement;
        }
    }

    /**
     * Toggles `focus` class to allow submenu access on tablets.
     */
    ( function() {
        var touchStartFn,
            parentLink = container.querySelectorAll( '.menu-item-has-children > a, .page_item_has_children > a' );

        if ( 'ontouchstart' in window ) {
            touchStartFn = function( e ) {
                var menuItem = this.parentNode;

                if ( ! menuItem.classList.contains( 'focus' ) ) {
                    e.preventDefault();
                    for ( i = 0; i < menuItem.parentNode.children.length; ++i ) {
                        if ( menuItem === menuItem.parentNode.children[i] ) {
                            continue;
                        }
                        menuItem.parentNode.children[i].classList.remove( 'focus' );
                    }
                    menuItem.classList.add( 'focus' );
                } else {
                    menuItem.classList.remove( 'focus' );
                }
            };

            for ( i = 0; i < parentLink.length; ++i ) {
                parentLink[i].addEventListener( 'touchstart', touchStartFn, false );
            }
        }
    }( container ) );
}() );

Any help is massively appreciated here