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How to change image element class with JavaScript based on window width?

I am trying to adjust the size of a background image based on the width of the window. I have been testing this, and I can get the alerts to show up in chrome when I 'inspect element' and change the width size, and the alerts show up as they should. But I cannot get the class of the image to change.
Any ideas?
This is my basefunctions.js file
window.onload = function changeClass(){
if( window.innerWidth < 770 ) {
document.getElementById("bg_img").setAttribute("class", "imgMobile");
alert("On Mobile");
}else{
alert("Not on Mobile");
}
}
This is my HTML/CSS
<script language="JavaScript" type="text/javascript" src="js/basefunctions.js"></script>
<style>
#bg_img {
width: 100%;
z-index: 1;
border: 1px #000 solid;
height:80%;
}
.imgMobile {
display: none;
width: 100%;
z-index: 1;
margin-left: -100;
}
</style>
<img src="img/gavel.png" alt="" id="bg_img" class="">

You should use className rather than using setAttribute.
document.getElementById("bg_img").className = "imgMobile";
Here is another SO about changing an dom object's class.
I also put together a jsfiddle to demonstrate.

You can set the class using
document.getElementById("bg_img").className = "imgMobile";
If you want to add the class without overriding other classes, then use
document.getElementById("bg_img").className += " imgMobile";

Before and After Slider with Multiple Images

I am trying to create a page that has before and after images that use a slider based on mouse movement to show both images. I need to have multiple sliders on the page and can not seem to get them to work. Below are a couple of different examples I have found and the challenges I am having.
http://codepen.io/dudleystorey/pen/JDphy - This works well with mobile but I can not seem to add a second version without adding css for every image since the background image is embedded in the css.
div#inked-painted {
position: relative; font-size: 0;
-ms-touch-action: none;
-webkit-touch-callout: none;
-webkit-user-select: none;
}
div#inked-painted img {
width: 100%; height: auto;
}
div#colored {
background-image: url(https://s3-us-west2.amazonaws.com/s.cdpn.io/4273/colored-panel.jpg);
position: absolute;
top: 0; left: 0; height: 100%;
width: 50%;
background-size: cover;
}
http://codepen.io/ace/pen/BqEer - Here is the other example that does not work as well with mobile. I can add the second image but the slider works all the images simultaneously and not individually when a second image is added.
Can anyone help with adding the second image. I am sure both of these are very workable but I am missing something in my css/javascript knowledge that is not allowing multiple images.

You need to loop though all classes to be able set the eventhandlers individual. Your codepen example could be change to this to work with individual images at once:
var blackWhiteElements= document.getElementsByClassName("black_white");
for (i = 0; i < blackWhiteElements.length; i++) {
initCode($(blackWhiteElements[i]));
}
function initCode($black_white) {
var img_width = $black_white.find('img').width();
var init_split = Math.round(img_width/2);
$black_white.width(init_split);
$black_white.parent('.before_after_slider').mousemove(function(e){
var offX = (e.offsetX || e.clientX - $black_white.offset().left);
$black_white.width(offX);
});
$black_white.parent('.before_after_slider').mouseleave(function(e){
$black_white.stop().animate({
width: init_split
},1000)
});
}
codepen here: http://codepen.io/anon/pen/mJPmKV

Your first attempt is near sufficient.
Assign the background-image inline in the html to avoid extra classes
<div id="colored" style="background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/4273/colored-panel.jpg);"></div>
change background-size on #colored to background-size: auto 100%; to reduce the "shaky" effect
background-size: auto 100%;

Change Background when part of it is hovered

i am totally new in web design, and i am right now struggling with creating part of my website, i need to somehow make this happen:
When PART of the BODY BACKGROUND is HOVERED, make the background change to "B", and when the mouse is not over that part, I need it to change back to background "A".
I have seen some examples here but as i am a beginner, i have no idea how to use javascript, if you could please give me some light here, either on pure CSS or on how to apply javascript.

This is accomplished very easily using a third party javascript library called JQuery http://jquery.com, you can see a working example here: http://jsfiddle.net/bbp8G/
$(document).ready(function(){
$("#hover").mouseenter(function(){
$(this).css("background","#009900");
}).mouseleave(function(){
$(this).css("background","#ffffff");
});
});

Here's the easiest way I know how to do what you've described...
<!-- POSITION THIS DIV WHEREVER YOU WANT THE
USER TO HOVER SO THAT THE BACKGROUND WILL CHANGE -->
<div id="hover">
</div>
<!-- PUT THIS CODE IN YOUR <HEAD> -->
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1.10.1/jquery.min.js" />
<style>
#hover { width: 200px; height: 200px; position: relative; top: 200px; background: green; }
.myNewBackround { background-color: red; }
</style>
<script>
$(document).ready(function() {
// when the #hover DIV is hovered, change the background of the body
$('#hover').hover(function() {
$('body').addClass('myNewBackground');
});
});
</script>
Here's a JS FIDDLE:
http://jsfiddle.net/ZKaJn/

Or you can do it with pure CSS
<div id="left"> </div>
<div id="right"> </div>
And the CSS part:
#left
{
background-color:#000;
float:left;
width:50%;
height:200px;
}
#right
{
background-color:#FF0;
float:right;
width:50%;
height:200px;
}
#right:hover
{
background-color:#00F;
}
#left:hover
{
background-color:#F00;
}
You can replace the div's and values with whatever you like, the main part is the #right:hover and #left:hover

Actually with just css it is not possible to change the background of the body when hovering a DOM element. This is because CSS does not allow you (yet) to travel up the DOM tree (select a parent), only down (select a child).
That being said, it is however possible to mimic the effect, and it is even quiet easy if it is the body background you want to change. You can lay a pseudo element with a background on top of your body background, and underneath the actual content. This way it looks as if the body background has changed.
The css to achieve this would look something like this:
.hover-me:hover:after {
content: '';
top: 0;
left: 0;
right: 0;
bottom: 0;
position: fixed;
background: url(http://placekitten.com/600/300) center center;
background-size: cover;
z-index: -1;
}
And a small fiddle to demonstrate: http://jsfiddle.net/3dwzt/
Should be compatible with IE8 and up

How to get css button to stay active after it has been clicked?

Im trying to make a nav bar where once the user clicks on the image, the image stays active. In the following example the leaf would stay green after is is clicked. Here is a bit of code of what Im talking about:
<a class="myButtonLink" href="#LinkURL">Leaf</a>
<style>
.myButtonLink {
display: block;
width: 100px;
height: 100px;
background: url('http://kyleschaeffer.com/wordpress/wp-content/uploads/2009/01/buttonleafhover.png') bottom;
text-indent: -99999px;
}
.myButtonLink:hover {
background-position: 0 0;
}
</style>

http://jsfiddle.net/bnaegele/XHBZf/2/
$('.myButtonLink').click(function() {
$(this).css('background-position', '0 0');
});

You could apply a class to it on click.
$('.myButtonLink').click(function() {
$(this).toggleClass('active');
});
This code has an added effect of deselecting the leaf on a second click. Depending on your requirements, you might want it or not.
Example: http://jsfiddle.net/stulentsev/XHBZf/1/

jquery slideshow using background images

I have a div which currently has a static background image.
I need to create a slideshow of background images for this div.
I am able to achieve this by just setting a timeout and then changing the background image in the CSS but this is not very elegant.
I would ideally like to fade the background images out and in, but the div contains other page elements so I can not alter the opacity in any way.
Does anyone know of a good way to do this using jquery??
Here's some code which fades out/in but fades out the contents of the div too.
$("#slideshow").fadeOut(5000, function(){
$("#slideshow").css('background-image','url(myImage.jpg)');
$("#slideshow").fadeIn(5000);
});

HTML:
<div class="slideshow"></div>
CSS:
.slideshow
{
position: relative;
width: 350px;
height: 150px;
}
.slideshow img
{
position: absolute;
width: 350px;
height: 150px;
z-index:-1;
}
jQuery
var images=new Array('http://placehold.it/250x150','http://placehold.it/250x150/123456','http://placehold.it/250x150/dbca98');
var nextimage=0;
doSlideshow();
function doSlideshow()
{
if($('.slideshowimage').length!=0)
{
$('.slideshowimage').fadeOut(500,function(){slideshowFadeIn();$(this).remove()});
}
else
{
slideshowFadeIn();
}
}
function slideshowFadeIn()
{
$('.slideshow').prepend($('<img class="slideshowimage" src="'+images[nextimage++]+'" style="display:none">').fadeIn(500,function(){setTimeout(doSlideshow,1000);}));
if(nextimage>=images.length)
nextimage=0;
}
jsfiddle Demo

How about adding a thumbs pagination list, to update the background image on click, and then, a second or two, and it starts fading in and out with the next bg img automatically?
HTML:
<div class="slideshow">
<h1>Text</h1>
<input type="button" value="Hello" />
</div>
<ul>
<li><img src="http://placehold.it/50x50"></li>
<li><img src="http://placehold.it/50x50/123456"></li>
<li><img src="http://placehold.it/50x50/dbca98"></li>
</ul>
CSS:
.slideshow
{
position: relative;
width: 350px;
height: 150px;
}
.slideshow img
{
position: absolute;
width: 350px;
height: 150px;
z-index:-1;
}
ul {position: absolute; top: 125px; left: 75px;}
li {
float: left;
border: 1px solid #000;
margin: 15px;
}
Javascript:
var images=new Array('http://placehold.it/250x150','http://placehold.it/250x150/123456','http://placehold.it/250x150/dbca98');
var nextimage=0;
doSlideshow();
function doSlideshow()
{
if($('.slideshowimage').length!=0)
{
$('.slideshowimage').fadeOut(500,function(){slideshowFadeIn();$(this).remove()});
}
else
{
slideshowFadeIn();
}
}
function slideshowFadeIn()
{
$('.slideshow').prepend($('<img class="slideshowimage" src="'+images[nextimage++]+'" style="display:none">').fadeIn(500,function(){setTimeout(doSlideshow,1000);}));
if(nextimage>=images.length)
nextimage=0;
}
See it all together at http://jsfiddle.net/tatygrassini/R4ZHX/75/.

Instead of just changing the background image, you could first call
fadeOut()
then change source, and then call
fadeIn()
something like...
$('#image').fadeOut(500, function() {
$(this).attr('src', 'new-image.png')
.load(function() {
$(this).fadeIn();
});
});
To use a variety of images, there are a number of solutions, but you could simply iterate through a list of them.

You can create an positioned absolutely and with a slider plugin change the images contained in the div. Otherwize you have to sprite the background. I achieved this with the Jquery Tools tabs plugin.
$(".slidetabs").tabs(".images > div", {
// enable "cross-fading" effect
effect: 'fade',
fadeOutSpeed: "slow",
// start from the beginning after the last tab
rotate: true
// use the slideshow plugin. It accepts its own configuration
}).slideshow();

Here is a solution that not only addresses your problem, but will also solve some other problems as well. Create another DIV on your DOM as an overlay, and execute your fade functions on this DIV only. It will appear as though the content is fading in / out. This approach is also more performant, as you are only fading a single DIV instead of multiple elements. Here is an example:
$('#containeroverlay').width($('#container').width()).height($('#container').height()).fadeIn('normal', function() {
// Step 1: change your content underneath the hidden div
// Step 2: hide the overlay
$('#containeroverlay').fadeOut('normal');
})
Most importantly, this approach will work in IE6-8 without screwing up the font aliasing of elements you may have on the div.