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In Chrome the currentSrc of an img element is sometimes empty. How can I prevent this?

UPDATE:
I just found some more info. It seems that in Chrome, the currentSrc of the image will often be empty, whereas in Firefox, the URL is always correct. Is the JS trying to access currentSrc before it's available? Is there a way to prevent this?
I am creating a Drupal 8 website, and in order to use the responsive images module with a background-image, I came up with the following workaround. The JS function below, setParallaxImage(), takes the currentSrc from the img in the picture element and sets it as the background-image for the outermost div. The img itself is not displayed (display: none in CSS) and is given a dummy image, as I only need it to get the currentSrc. The function is called with onload and onresize.
The code seems to work well in Firefox. When resizing the browser past a breakpoint, the image goes grey for a split second, then loads the image from the proper source. However, with Chrome, when quickly resizing past a breakpoint, the image may become grey and not get displayed at all, that is, background-image: url(). Usually if I resize the window a couple more times, the image will finally appear. Does anyone know why this might be happening? Thank you for reading.
JS
function setParallaxImage() {
const parallaxContainer = document.getElementsByClassName('paragraph--type--parallax-banner-with-text');
for(var i = 0; i < parallaxContainer.length; i++) {
console.log('in setparallax');
var x = parallaxContainer[i];
var parallax = x.getElementsByClassName('field--name-field-parallax-image-top-')[0];
var picture = parallax.getElementsByTagName("picture")[0];
var sourceURL = picture.querySelector('img').currentSrc;
x.setAttribute('style', 'background-image: url(' + sourceURL +')');
var img = picture.getElementsByTagName("img")[0].setAttribute('src', '/sites/default/files/src_images/dummy_image.gif');
}
}
window.onload = setParallaxImage
window.onresize = setParallaxImage;
HTML
<div class='paragraph--type--parallax-banner-with-text'>
<div class='field--name-field-parallax-image-top-'>
<picture>
<!-- several source tags here -->
<img src="will-be-given-dummy-image-in-JS">
</picture>
</div>
</div>

You can use the property "complete" to check, if the image was already loaded. If not, u can use the onload-Event to fire, when the image is available.
if (img.complete) {
console.log(img.currentSrc);
} else {
img.onload = function() {
console.log(img.currentSrc);
}
}

How to cancel a background image from loading

How can you cancel the loading of an image defined by a background-image attribute?
There a few questions that show how to cancel the loading of an <img> tag by setting the src to '' or to a different image (such as this one, answered by Luca Fagioli). But this does not work for background images.
Luca provided a jsfiddle ( jsfiddle.net/nw34gLgt/ ) to demonstrate the <img src="" /> approach to canceling an image load.
But modifying that jsfiddle to use background-image instead clearly shows that the image continues loading, even if:
you do background-image: none (as suggested here)
or background-image: url("web.site/other_image.jpg")
or background-image: url('').
In fact, in my testing on Firefox 54, the background image continues loading even if you do window.stop() or close the tab.
So, is there any way at all to stop loading a background image once it starts?
My use-case for this is client-side, so I can't change the site to not use background images. I am viewing a gallery of many thumbnail images, but the thumbnails are much larger than they need to be. Smaller versions are available so I wanted to replace the large thumbnails with the smaller versions via Greasemonkey to ease the network load on my poor, slow connection. Each entry in the gallery is a <div> with a background-image inside an <a> linking to the full-size image. (using Fotorama).

If you need to target inline styles on the elements, then I think you can run a script near the bottom of the page to do that.
Testing this locally, the original images do not show as loading in the network tab.
var allDivs = document.getElementsByTagName('DIV');
for (var i = 0; i < allDivs.length; i++) {
// check for inline style
var inlineStyle = allDivs[i].getAttribute('style');
// check if background-image is applied inline
if (inlineStyle && inlineStyle.indexOf('background-image') != 1 ) {
allDivs[i].style.backgroundImage = 'url("newImg.jpg")'; // assign new value?
}
}
That is going to grab every div, so hopefully these elements have a class you can use to query first. Then you have a smaller collection to run the above on.
Classes, not inline styles:
If you could target classes, it would look a bit cleaner. You could create a new class and use it.
var els = document.querySelectorAll('.bg-img');
for (var i = 0; i < els.length; i++) {
els[i].classList.remove('bg-img');
els[i].classList.add('no-bg-img');
}

Improve image load time

I have a few img tags whose src is changed onmouseover. This takes an inordinate amount of time to load. How can I improve the load time? The images are basically just different icons.

You can do a few things.
1) CSS Sprites is probably the preferred method.
2) You can load the images in a div and set that div to display none, making it so the images are already loaded so on mouseover they'll be there instantly.
Also here's a link on how to PreLoad images with CSS

There are a few ways to do it, the ideal solution in your case would be to use CSS sprites considering they're icons. However, depending on the situation sometimes sprites aren't ideal.
Here's one solution using JavaScript to preload images:
var images = new Array();
function preload() {
for (i = 0; i < preload.arguments.length; i++) {
images[i] = new Image();
images[i].src = preload.arguments[i];
}
}
preload(
'http://image-1.jpg',
'http://image-2.jpg',
'http://image-3.jpg'
);

What you're trying to do is achieve a rollover. It is strange that you'd experience a very long delay in this process. Usually, if the images aren't stored in some remote location, they're pretty fast.
Look at this article for some guidance
Other things you could try:
- sprites in css
- you could use two overlapping divs and hide one and unhide the other and vice versa

JavaScript How to make an image to change color onmouseover?

I have a button and an image and want them to change color onmouseover.
The button changes color fine:
<script>
function secondColor(x) {
x.style.color="#000000";
}
function firstColor(x) {
x.style.color="#ffaacc";
}
</script>
<input onmouseover="secondColor(this)" onmouseout="firstColor(this)" type="submit"><br>
How can I do the same thing with the image? Is there any way:
<img src="..." ......
Or do I have to have a second image to replace the first one onmouseover and this is the only way?

If you don't care that much about supporting older browsers, you could use the new CSS3 filter brightness. In chrome, you could write something like this:
var image = document.getElementById('img');
image.addEventListener('mouseover', function() {
image.setAttribute('style','-webkit-filter: brightness(1.5)');
}, false);
image.addEventListener('mouseout', function() {
image.setAttribute('style','-webkit-filter: brightness(1.0)');
}, false);
I don't recommend this approach, though. Using another picture while hovering would be a better solution.

I know that this is old, but you don't need two images. Checkout my example using one image.
You can simply change the position of the background image.
<div class="changeColor"> </div>
JavaScript
var dvChange = document.getElementsByClassName('changeColor');
dvChange[0].onmouseover = function(){
this.style.backgroundPosition = '-400px 0px';
}
dvChange[0].onmouseout = function(){
this.style.backgroundPosition = '0px 0px';
}
CSS
.changeColor{
background-image:url('http://www.upsequence.com/images/multibg.png');
width:400px;
height:400px;
background-position: 0px 0px;
}
.changeColor:hover{
background-image:url('http://www.upsequence.com/images/multibg.png');
width:400px;
height:400px;
background-position: -400px 0px;
}
You can also try changing the opacity of the images onmouseover and onmouseout.
I don't have an example for that, but its super easy to find and I am sure it has be answered already on stack exchange somewhere.
In the JSFiddle below there is Javascript and non-Javascript examples.
http://jsfiddle.net/hallmanbilly/gtf2s8ts/
Enjoy!!

I think you have to use a second image. I recently cam across the following article describing how to do image crossfading on hover using css. Crossfading Image Hover Effect

You can change image SRC on mouse over, you can load two images and use fade effects to "change" them. But better, you can use image as DIV background, make sprite and just move BG on mouse over.
Loading of two different images bring you to disappearing when hover and second image loading. Better do not use JS at all. Make sprite from two images, put it as BG of DIV and write two CSS for DIV, normal and when hover.

If you have access to JQuery use hover function. If you want to change image
$('#imageid').hover(function(){
//change image or color or opacity
$(this).attr('src', newImageSrc);
});
add this function in document ready function.

Remove images from UIWebview

In my iPhone app, an epub reader, based off the method here, I have parsed the epub, created the UIWebviews, but I have a slight problem. There are images in the epubs that are larger than the width of the iPhone's screen (320 px.). Is there a Javascript method I can invoke on the UIWebview ([view stringByEvaluatingJavaScriptFromString:SomeJavaScriptString]) and remove those images programatically without changing the epub manually?
UPDATE: Could the issue be that the source file is an xml file and not HTML?

You probably want something like: document.getElementById('id_of_your_image').style.visibility = 'hidden'
UPDATE
To hide all images in a document,
for (i=0; i<document.getElementsByTagName("img").length; i++) {
document.getElementsByTagName("img")[i].style.visibility = 'hidden';
}
should do the trick.

var images = Array.prototype.slice.call(document.getElementsByTagName('IMG'), 0);
var imageCount = images.length;
for (var i = 0; i < imageCount; i++) {
var image = images[i];
image.parentNode.removeChild(image);
}

If you're not dealing with graphs or images where seeing the detail is important why not:
1) write some CSS that sets a max-width on images of 320px or 640px, depending on orientation.... see http://www.thecssninja.com/css/iphone-orientation-css for how to use different css for different orientations.
2) insert that CSS in each HTML file after any other styles or stylesheet links - inside a <style> element added just before the </body> would work.
?
That way images are still visibile but won't extend past a single page width.
As a next step you could wrap images with something like <a href='zoom://x'> where x is the image filename - then in your UIWebViewDelegate' "shouldStartLoadWithRequest" function check to see if request.URL.scheme == "zoom" - if it is you could push a new view containing image x in a zoomable container, like iBooks.