Why does my function fail intermittently?

Question

I need some help with this script. Can't find out why it fails sometimes. It is working most of the time, but sometimes it's just showing blank page.

This is the script (in the folders I have 3 image files named 1.jpg, 2.jpg and 3.jpg.:

function chgbg() {
    var d = new Date();
    var h = d.getHours();
    var n = d.getDay();
    var totalCount = 3;
    var num = Math.ceil(Math.random() * totalCount);

    /* Monday - Friday */
    if (n >= 1 && n < 5 && h >= 23 && h < 6) {
        document.body.style.backgroundColor = "#FFFFFF";
        document.body.style.backgroundImage = 'url(gfx/weekdays/00-06/' + num + '.jpg)';
    }
    if (n >= 1 && n < 5 && h >= 6 && h < 11) {
        document.body.style.backgroundColor = "#FFFFFF";
        document.body.style.backgroundImage = 'url(gfx/weekdays/06-11/' + num + '.jpg)';
    }
    if (n >= 1 && n < 5 && h >= 11 && h < 14) {
        document.body.style.backgroundColor = "#FFFFFF";
        document.body.style.backgroundImage = 'url(gfx/weekdays/11-14/' + num + '.jpg)';
    }
    if (n >= 1 && n < 5 && h >= 14 && h < 18) {
        document.body.style.backgroundColor = "#FFFFFF";
        document.body.style.backgroundImage = 'url(gfx/weekdays/14-18/' + num + '.jpg)';
    }
    if (n >= 1 && n < 5 && h >= 18 && h < 23) {
        document.body.style.backgroundColor = "#FFFFFF";
        document.body.style.backgroundImage = 'url(gfx/weekdays/18-00/' + num + '.jpg)';
    }

    /* Monday - Friday */

Answer 1

Your first condition is impossible

if (n >= 1 && n < 5 && h >= 23 && h < 6)

h can't be both >=23 and <6 at the same time.

Answer 2

Surely it will fail on the first condition (when your hour is between 23 and 6), because you are telling javascript "if hour is equal to or greater than 23 and less than 6", and this isn't possible.

if (n >= 1 && n < 5 && h >= 23 && h < 6)

A solution could be

if (n >= 1 && n < 5 && (h >= 23 || (h >= 0 && h < 6)))

Answer 3

FYI, in cases like this one I really like to use loops, which iterate over arrays of structures. In this example, each of the cases appear to differ only by: the boundary values for n and h , and a substring that will be inserted into the backgroundImage URL. So, your code loops through this structure and, as soon as it finds a matching case, it uses break to get out. Much more maintainable.