Reusing a jQuery function without causing an error on pages that do not utilize it

I use this function to make a help box stay visible when a user scrolls down:

var top = $('.help-box').offset().top - parseFloat($('.help-box').css('marginTop').replace(/auto/, 0));
$(window).scroll(function (event) {
    var y = $(this).scrollTop();
    if (y >= top) {
        $('.help-box').addClass('fixed')开发者_运维知识库;
    } else {
        $('.help-box').removeClass('fixed');
    }
});

I want to reuse it across several pages, so I included it in my layout (on every page load). The problem now is that I get an error on pages that do not have the help box: $(".help-box").offset() is null

Is there a way to write this function so it can be reused without causing an error? I want to avoid selectively including where it's need it as it's easier to just leave the include in my layout.

var helpBox = $(".help-box"), top;
if(helpBox !== null && helpbox != undefined && helpBox.length > 0) {
    top = helpBox.offset();
}

Just check whether you have a helpbox before calling the function.

Put everything in try-catch block:

try
{
var top = $('.help-box').offset().top - parseFloat($('.help-box').css('marginTop').replace(/auto/, 0));
$(window).scroll(function (event) {
    var y = $(this).scrollTop();
    if (y >= top) {
        $('.help-box').addClass('fixed');
    } else {
        $('.help-box').removeClass('fixed');
    }
});
}
catch(err)
{
}

    var helpBox = $('.help-box');

    function initHelpBox(){
      var top = helpBox.offset().top - parseFloat(helpBox.css('marginTop').replace(/auto/, 0));
    $(window).scroll(function (event) {
        var y = $(this).scrollTop();
        if (y >= top) {
            helpBox.addClass('fixed');
        } else {
            helpBox.removeClass('fixed');
        }
    });
    }

 if (helpBox.length){initHelpBox()}