I'm trying to set the width of an html element when a menu button is clicked.

    m('div.menuButton', {onclick: 
            function () {
                var elNav = document.getElementById('sideNav');  
                if (elNav.style.width = '0') {
                    elNav.style.width = '250';
                } else { 
                    elNav.style.width = '0';
                }    
            }
        }, 
        m('img.menuButton', {src: MENU_BUTTON}),
    ) 

But it just refreshes the page and doesn't make the element width wider. If i add m.render(); it will work but then i get an error in console saying DOM element does not exist.

I've tried googling the issue but come up with nothing. Can anyone help?