JavaScript, How to change the background of a div tag every x seconds

Question

I'm trying to make some JavaScript code to change the background of two div tags every X seconds. Here is my code:

HTML

<div id="bg_left"></div>
<div id="bg_right"></div>

CSS body{ height:100%; }

#bg_left{
   height:100%;
   width:50%;
   left:0;
   position:fixed;
   background-position:left;
}
#bg_right{
    height:100%;
    width:50%;
    right:0;
    position:fixed;
    background-image:url(http://presotto.daterrawebdev.com/d/img/pp_hey_you_bg.png);
    background-position:right;
}  

JAVA SCRIPT

 function carousel_bg(id) {
     var bgimgs = [  'pp_hey_you_bg.png', 'burningman_bg.png' ]; 
     var img1 = bgimgs[id];
     var img2 = bgimgs[id+1];
     var cnt = 2; 

     $('#bg_left').css("background-image", "url(http://presotto.daterrawebdev.com/d/img/"+img1+")");
     $('#bg_right').css("background-image", "url(http://presotto.daterrawebdev.com/d/img/"+img2+")");

     id = id + 1;
     if (id==cnt) id = 0;

     setTimeout("carousel_bg("+id+")", 10000); 
 }

 $(document).ready(function() {
        carousel_bg(0);     
 });

​ The background-images should be changing randomly, but they don't even change at all.

Answer 1

OK, I see the issue in your jsFiddle. Because you're passing a string to setTimeout() that string will be evaluated only at the top level scope. But, the function name you were passing is not at the top level scope (it's in an onload handler for the jsFiddle). So, I changed the way your JS is positioned in the jsFiddle so it is now at the top level scope. I also fixed up the logic for selecting an image and it now works here: http://jsfiddle.net/jfriend00/awVYP/

And, here's a cleaned up version that does not pass a string to setTimeout() (a much better way to write javascript) that passes a local function and uses a closure to keep track of the current index: http://jsfiddle.net/jfriend00/LVGNN/

    function carousel_bg(id) {
        var bgimgs = [  'pp_hey_you_bg.png', 'burningman_bg.png' ]; // add images here..

        function next() {
            if (id >= bgimgs.length) {
                id = 0;
            }
            var img1 = bgimgs[id];
            id++;
            if (id >= bgimgs.length){
                id = 0;
            }
            var img2 = bgimgs[id];

            $('#bg_left').css("background-image", "url(http://presotto.daterrawebdev.com/d/img/"+img1+")");
            $('#bg_right').css("background-image", "url(http://presotto.daterrawebdev.com/d/img/"+img2+")");
            setTimeout(next, 1000);
        }
        next();
    }

    $(document).ready(function() {
            carousel_bg(0);     
    });

---

Previous comments on earlier version so of the OP's code:

$('#body')

should be:

$('body')

or even faster:

$(document.body)

Also, your jsFiddle shows a bit of an odd issue. Your CSS has a background image on the HTML tag, but your javascript sets a semi-transparent background image on the body tag. Is that really what you want?

Answer 2

For testing I added another image to the array so that we got some distinction in the sorting.

function carousel_bg(id) {
    var bgimgs = [  'http://presotto.daterrawebdev.com/d/img/pp_hey_you_bg.png', 'http://presotto.daterrawebdev.com/d/img/burningman_bg.png', 'http://gallery.orobouros.net/var/albums/2012/NewYorkComicCon2012/Legend-of-Korra/nycc_20121013_164625_0041.jpg?m=1354760251' ]; // add images here..
    var img1 = bgimgs[id+1];
    var img2 = bgimgs[id];
    var cnt = bgimgs.length; // change this number when adding images..

    $('#bg_left').css("background-image", "url("+img1+")");
    $('#bg_right').css("background-image", "url("+img2+")");

    id = id + 1;
    if (id== (cnt - 1) ) id = 0;

    setTimeout("carousel_bg("+id+")", 10000);
}

Two changes here:

1. For your total image count, I am retrieving the total count of images in the array dynamically instead of by hand ( bgimgs.length )
2. In your conditional to reset the id value, subtract the total count by 1. Since JS has zero-based indexes, not doing this will get you an undefined error (a 3 item array will spit out a value of 4 in your original code on the last iteration).

While this code does loop through your array, it's not random. That's another topic.

Answer 3

对于不使用JQuery的用户，只需执行以下操作：

document.body.style.backgroundImage="url(images/mybackgroundimage.jpg)";