Uncaught TypeError: Cannot read property 'clientWidth' of null

try adding your script at the end of the document.The reason is that, a the beginning your document width is zero, because you content is not loaded yet, so there's nothing to display and so, you width is zero

I would say use $(window).width(); from jquery. It returns width of browser viewport Which is equivalent or I would say better alternative for traditional javascript.

Your code will look something like this. Check if it works.

var ww = $(window).width();$(document).ready(function() {    adjustMenu();    $(".cat").click(function(e) { // cat class        e.preventDefault();        $(this).toggleClass("active");        $(".sf-menu").toggle();    });});function adjustMenu() {    if (ww <= 740) { //change this to your breakpoint        $('.sf-menu').hide();        $(".cat").show();        if (!$(".cat").hasClass("active")) {            $(".sf-menu").hide();        } else {            $(".sf-menu").show();        }    } else {        $('.sf-menu').show();        $(".cat").hide();    }}$(window).bind('resize orientationchange', function() {    ww = document.body.clientWidth;    adjustMenu();    });    function adjustMenu() {        if (ww <= 740) { //change this to your breakpoint        $('.sf-menu').hide();        $(".cat").show();        if (!$(".cat").hasClass("active")) {            $(".sf-menu").hide();        } else {            $(".sf-menu").show();        }    } else {        $('.sf-menu').show();        $(".cat").hide();    }}$(window).bind('resize orientationchange', function() {    ww = $(window).width();    adjustMenu();});