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How to change a background that is declared in .css not html

USING JAVASCRIPT, NO JQUERY
Hi all,
I know this is a basic one but I am hitting a dead end.
I want to change a background image that is in the .css, not the html so I cant give it an id. I managed to remove the image using:
var headerImg = document.getElementById('header').background = 'none';
And tried :
var headerImg = document.getElementById('header').background = 'images/new-header.jpg;
But that did'nt work.
I have no idea how to change the Image, and in the dev tools the url does not even change when I try to run my code Any help would be great, Thanks.

You're close. You're just off on the syntax slightly...
document.getElementById("header").style.backgroundImage = "url(images/new-header.jpg)";
It's a style attribute you're changing, so you need .style and then you use the CSS attribute name, but remove hyphens and camelCase the attribute name, so .backgroundImage.

Can you try this
var element = document.getElementById('content');
element.style.backgroundImage = "url('https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSzOpUTzDIq2yutn75PQTLcHhJ06MTZPsV_V-0M918xKUhAqMxE')";
<div>
<p>this is unchanged</p>
<p id="content">backgroud wiil be cange</p>
<div>

Jquery replaceWith keep onclick

How can I keep onclick="" value with JQuery replaceWith ? I'm making a assets system that preload every image and put it on a Javascript image() object, and using a special data attribute for img urls
<img data-assets="images/test.png" onclick="alert('test')">
turn into : (using jquery replaceWith)
<img src="assets/images/test.png">
What I want:
<img src="assets/images/test.png" onclick="alert('test')">
My code:
$("[data-assets]").each(function() {
$(this).replaceWith(Game.Preloading.Assets.Images[$(this).data('assets')]);
});
How can I fix that ? Thanks

While iterating over each [data-assets] element, you could set the corresponding onclick attribute before replacing the element:
$("[data-assets]").each(function() {
var $newImg = $(Game.Preloading.Assets.Images[$(this).data('assets')]);
$newImg.attr('onclick', $(this).attr('onclick'));
$(this).replaceWith($newImg);
});
However, it would be better to just add a src attribute on the existing element rather than replacing it:
$("[data-assets]").each(function() {
this.src = $(Game.Preloading.Assets.Images[$(this).data('assets')]).attr('src');
});
Ideally, you should be using unobtrusive JavaScript and avoiding the inline JavaScript event listeners, but both of the above snippets should work.

I think you would be better off to simple query for your attribute, then use the each method to update the SRC attribute on each matched element.
Im on my phone so a more detailed answer is difficult...
But here goes
$("[data-assets]").each(function(){ $(this).attr("src", Game.Preloading.Assets.Images[$(this).data('assets')]); });

Get image src without loading the image?

I have a html snippet being returned through ajax. The snippet is an <img> tag.
<img src="image.jpg" />
I need to extract the value of the src attribute without loading the image initially. The reason for this is the src attribute contains a partial path that I need to manipulate in my app to load the image properly.
I have the following code currently extracting the src attribute:
var src = $(value).attr('src');
However, this causes the image to load, resulting in a 404. I would like to avoid this unnecessary request.
How can I extract the value of the src attribute without causing the browser to load the image?

I solved this by changing the name of the src attribute before loading it into jquery.
value = value.replace('src', 'data-src');
var src = $(value).attr('data-src');
Doing this allows me to extract the value without causing the browser to attempt to load the images.

Your best bet is probably to output a data tag on the image. You can then manipulate this using jQuery and then use it to write the final image path.
So you'd do something like this in your HTML:
<img data-img-src="abc.jpg" class="my-image" />
Then in your jQuery:
var path = $('.my-image').data('img-src');
console.log(path); // Do something here, then write the new path with:
$('.my-image).attr('src', new_path);
EDIT: Sorry I just re-read the bit where it's coming via AJAX. In that case, you can probably use the data callback of your ajax request to strip the src from the image.

$.ajax('someURL.html', function(data){
var html = data.replace(/\ssrc/g, ' data-src'),
img = $(html),
src = 'some/path/' + img.data('src');
img.attr('src', src);
$('#container').append(img);
});

If you just have the string , like <img src="image.jpg" /> why dont you go for regex?
Something like: /[\"\'][a-z0-9A-Z\.]*/.
PS:My regex skills are poor,so you could manipulate it accordingly.

Use
var string = '<img src="image.png">';
var matches = string.match(/src\=("|')(.*?)\1/);
console.log(matches[2]);

You can simply remove the attribute after accessing it.
This will not load the invalid image, as you can confirm in your console:
var s= $('<img src="invalidURL.jpg">'),
src= s.attr('src');
s.removeAttr('src');
console.log(src);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
Remove removeAttr(), and it will attempt to load the image.

Change attribute from data-src to src WITHOUT jQuery

I am building a lightbox, in pure JavaScript. I currently have my images loading via AJAX, but i know it is easier to just have the img data-src attribute being replaced onclick to src.
I however have NO idea how this is done in pure JavaScript, by that i mean, without using any libraries.
Can anyone tell me how this is done?
to sum up: How do i change ex:
<img data-src="URL"/>
to:
<img src="URL"/>
without jQuery.

You can do it like shown below:
var imgEl = document.getElementsByTagName('img');
for (var i=0; i<imgEl.length; i++) {
if(imgEl[i].getAttribute('data-src')) {
imgEl[i].setAttribute('src',imgEl[i].getAttribute('data-src'));
imgEl[i].removeAttribute('data-src'); //use only if you need to remove data-src attribute after setting src
}
}
The above code will fetch all img tags, check if they have a data-src attribute and if present, replace it with src.
Demo Fiddle

Get a handle on the image element, and then set it's src property, using the value from getAttribute().
Plain Javascript doesn't have any helper functions to handle data-* attributes, it just treats them as any other attribute.
var img = document.getElementById("myimg");
img.src = img.getAttribute("data-src");

You can use forEach with querySelectorAll
var imageElements = document.querySelectorAll('img');
imageElements.forEach(c=> c.setAttribute('src',c.getAttribute('data-src')));
img{width:100px;}
<img data-src='https://cdn4.iconfinder.com/data/icons/nature-20/512/79-512.png' />

Setting a JS variable and using it in html tag

I have set a variable, and I need to pull in this variable into a html element, but I cant get it to print out the value, this is the code:
<script>
var randomnumber=Math.floor(Math.random()*11)
</script>
<div id="<script type="text/javascript">document.write(randomnumber)</script>"></div>
Thanks.
Edit: I just used a div as an example, but i need to add a random number to an img tag, as it is for a tracking tag, and needs a unique identifier. Is there a better way to go about this?

Use
<script>
document.write('<div id="'+randomnumber+'" ></div>');
</script>
You can't open a script tag inside an attribute

Or you can create it using JS:
var randomnumber=Math.floor(Math.random()*11);
a = document.createElement('div');
a.setAttribute('id',randomnumber);
document.body.appendChild(a);
// if you know the exact class or ID where it is to be appended you can use
document.getElementsByClassName("myclass")[0].insertBefore(a, document.getElementsByClassName("beforeClass").firstChild);
This will create a div, with the id as the randomnumber and append it to the body