Develop Reference

How to lazy load images and make them available for print

Some websites have lots of images, so lazyloading seems appropiate to reduce load times and data consumption. But what if you also need to support printing for that website?
I mean, you can try to detect the print event and then load the images, with something like this:
HTML
<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7">
Note: this is a one by one pixels gif dummy image.
JavaScript
window.addEventListener('DOMContentLoaded', () => {
img = document.querySelector('img');
var isPrinting = window.matchMedia('print');
isPrinting.addListener((media) => {
if (media.matches) {
img.src = 'http://unsplash.it/500/300/?image=705';
}
})
});
Note: if you try this in a code playground, remove the DOMContentLoaded event (or simply fork these: JSFiddle | Codepen).
Note: I didn't event bother with the onbeforeprint and onafterprint for obvious reasons.
This will work fine if the image is cached, but then again that's precisely not the point. The image/s should all load and then appear in the printing screen.
Do you have any ideas? Has anyone successfully implemented a print-ready lazyloading plugin?
Update
I've tried redirecting the user after the print dialog is detected, to a flagged version of the website a.k.a website.com?print=true where lazyloading is deactivated and all images load normally.
This method is improved by applying the window.print() method in this flagged print-ready version of the page, opening a new print dialog once all images are finished loading, and showing a "wait for it" message in the meantime at the top of the page.
Important note: this method was tested in Chrome, it does not work in Firefox nor Edge (hence why this is not an answer, but a testimony).
It works in Chrome beacuse the print dialog closes when you redirect to another website (in this case same url but flagged). In Edge and Firefox the print dialog is an actual window and it does not close it, making it pretty unusable.

Based on your desired functionality, I'm not quite sure what you want to do is feasible. As a developer we don't really have control over a users browser. Here are my thoughts as to why this isn't fully possible.
Hooking the event to go and load your missing images won't let you guarantee images will make it from the server into your page. More specifically, the PDF generated for your print preview is going to get generated before your image(s) is done loading, the img.src = "..." is asynchronous. You'd run into similar issues with onbeforeprint as well, unfortunately. Sometimes it works, sometimes it does not (example, your fiddle worked when testing in safari, but did not in Chrome)
You cannot stall or stop the print call -- you can't force the browser to wait for your image to finish loading in the lazy loading context. (I read something about using alerts to achieve this once, but it seemed really hacky to me, was more of a deterrent to printing than stalling)
You cannot force img.src to get that data synchronously in a lazy-load context. There are some methods of doing this, but they are clever hacks -- referenced as pure evil and may not work in always. I found another link with a similar approach
So we have a problem, if the images are not loaded by the time print event is fired, we cannot force the browser to wait until they are done. Sure, we can hook and go get those images on print, but as above points out, we cannot wait for those resources to load before the print preview pops up.
Potential solution (inspired by links in point three as well as this link)
You could almost get away with doing a synchronous XMLHttpRequest. Syncrhonous XMLHTTPRequests will not let you change the responseType, they are always strings. However, you could convert the string value to an arrayBuffer encode it to a base-64 encoded string, and set the src to a dataURL (see the link that referenced clever hacks) -- however, when I tried this I got an error in the jsfiddle -- so it would be possible, if things were configured correctly, in theory. I'm hesitant to say yes you can, since I wasn't able to get the fiddle working with the following (but it's a route you could explore!).
var xhr = new XMLHttpRequest();
xhr.open("GET","http://unsplash.it/500/300/?image=705",false);
xhr.send(null);
if (request.status === 200) {
//we cannot change the resposne type in synchronous XMLHTTPRequests
//we can convert the string into a dataURL though
var arr = new Uint8Array(this.response);
// Convert the int array to a binary string
// We have to use apply() as we are converting an *array*
// and String.fromCharCode() takes one or more single values, not
// an array.
var raw = String.fromCharCode.apply(null,arr);
// This is supported in modern browsers
var b64=btoa(raw);
var dataURL="data:image/jpeg;base64,"+b64;
img.src = dataURL;
}
Work around to enhance the user experience
Something you could do is have some text that only displays in the print version of your page (via #print css media) that says "images are still loading, cancel your print request and try again" and when the images are finished loading, remove that "still waiting on resources try again message" from the DOM. Farther, you could wrap your main content inside an element that inverses the display to none when content is not loaded, so all you see is that message in the print preview dialog.
Going off of the code you posted this could look something like the following (see updated jsfiddle):
CSS
.printing-not-ready-message{
display:none;
}
#media print{
.printing-not-ready-message{
display:block;
}
.do-not-print-content{
display:none;
}
}
HTML
<div class="printing-not-ready-message">
Images are still loading please cancel your preview and try again shortly.
</div>
<div class="do-not-print-content">
<h1>Welcome to my Lazy Page</h1>
<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7">
<p>Insert some comment about picture</p>
</div>
JavaScript
window.addEventListener('DOMContentLoaded', () => {
img = document.querySelector('img');
var isPrinting = window.matchMedia('print');
isPrinting.addListener((media) => {
if (media.matches) {
img.src = 'http://unsplash.it/500/300/?image=705';
//depending on how the lazy loading is done, the following might
//exist in some other call, should happen after all images are loaded.
//There is only 1 image in this example so this code can be called here.
img.onload = ()=>{
document.querySelector(".printing-not-ready-message").remove();
document.querySelector(".do-not-print-content").className=""
}
}
})
});

I'm the author of the vanilla-lazyload script and I've recently developed a feature that makes print of all images possible!
Tested cross browser using this repo code which is live here.
Take a look and let me know what you think!
I'm open to pull requests on GitHub of course.

I wrote a lazy loading jquery plugin that supports showing images on print using the window.onbeforeprint events and mediaQueryListeners.
https://github.com/msigley/Unveil-EX/

//declare custom onbeforeprint method
const customOnBeforePrint = () => {
const smoothScroll = (h) => {
let i = h || 0;
if (i < 200) {
setTimeout(() => {
window.scrollTo(window.scrollY, window.scrollY + i);
smoothScroll(i + 10);
}, 10);
}
};
let height = document.body.scrollHeight;
let newHeight;
while (true) {
smoothScroll(100);
if (newHeight === height) break;
height = newHeight;
}
};
//override the onbeforeprint method
window.onbeforeprint = customOnBeforePrint;
Copy&Paste that block into devtool's console and then try to click print button. That workaround is working for me.

For whoever is in the same boat as I was: when using the browser native loading="lazy", you can simply remove that attribute when printing is going to happen. Below is my jQuery implementation.
window.onbeforeprint = function () {
$('img').each(function () {
$(this).removeAttr('loading')
});
}
Chrome will then just load all images and they will show up when printing.

Once an image has failed loading, how can I make the browser retry?

I'm making a Chrome extension which in order to reduce bandwidth usage it stops all outcoming requests which are images.
I want to provide functionality where if the user clicks on the image (or technically a layer on top of that image) it would try to reload the image, this time not being blocked by the extension.
How can I tell the browser to retry loading the image? And if there isn't a straightforward way to do it, what would be a work around? Deleting the old image from the DOM and adding it again?
Any help is appreciated. :)
EDIT 1:
To answer #CBroe's question:
Using the chrome.webRequest.onBeforeRequest API in a background script.
To answer #jfriend00's question:
The usual placeholder "couldn't load image" icon, I guess also known as "broken file" icon:
See all those broken images?
That screenshot also illustrates the point of a layer on top of another image. Should those images not be broken, the loaded image would be there but that layer (the one in a dark grey which shows the image's dimensions) still remains there.
The desired href still exists there in the img tag:

If simply assigning the same src value to the img element is not enough¹, then create a new Image object in JavaScript, and assign the value to its src property.
¹ It might not be, if the browser just goes, “oh hey, that is the same value for the src attribute that the img already had, so I don’t have to do anything” – creating a new JS Image object however should make the browser request that resource again if he realizes he does not have it cached already.

What I would do instead is replace the URLs of the images with an image from your extension. A 1x1 pixel transparent GIF or PNG.
When you do this, add an attribute to all of the elements you replaced... something like data-yourextension-originalurl, with the URL of the original image. If the user then wants to load images, it's easy enough to go back and fix those image elements.

While I'm not too familiar with the Chrome API, a quick glance seems to suggest that there's no way to get the specific img element from each onBeforeRequest, which you'd need to know in order to figure out where to attach custom code.
This may be better accomplished with native JavaScript of some sort. For example, if Chrome lets you inject code on load, you could apply a function like the one below to all img elements after document load but before image load.
// Given an img element, replaces its src with a placeholder URL,
// and sets its click action to load its original src
function makePlaceholder(elem){
elem["data-oldtitle"] = elem.title;
elem["data-oldhref"] = elem.href;
elem["data-oldsrc"] = elem.src;
elem["data-oldonclick"] = elem.onClick;
elem.title = "Click to load the blocked image.";
elem.href = '';
elem.src = "http://example.com/placeholder.png";
elem.onClick = function(){
this.src = this["data-oldsrc"];
this.title = this["data-oldtitle"];
this.href = this["data-oldhref"];
this.onClick = this["data-oldonclick"];
};
}

The simple way to force reloading an image in JavaScript is:
var img = document.getElementById("myImage");
img.src = img.src.replace(/\?.+/,"") + "?" + new Date().getTime();
This adds a unique QueryString to the image which basically forces the browser to not use a cached version of the image.

Gif loader while loading heavy png image

I load png image generated by server-side PHP script (chart) to the HTML IMG-element <img id="chart"> using following JS code:
$('#chart').attr('src', 'chart.php');
The PNG-image generation and downloading takes about 1 second, so I want to show gif loader while image is loading. How to implement this feature with JS?

For balance this is very simple to do in plain JS:
var preload = function(element, src) {
var img = new Image();
// Apply onload before applying src attribute to avoid IE prematurely firing
img.onload = function() {
// Replace #chart with image
element.parentNode.replaceChild(img, element);
};
img.src = src;
}
preload(document.getElementById('chart'), 'chart.php?_...');

$('#chart').attr('src', 'chart.php').load(function(){
//something
});

In case the browser caches it, you may way to add something to the query string to break that. Either way, you need to listen for the image's load event, which should be bound before setting its src (in case it's cached):
var target_url = 'chart.php?_=' + (new Date()).getTime();
// Show "loading"
$('#chart').on("load", function () {
// Hide "loading"
}).attr('src', target_url);
Reference:
http://api.jquery.com/load-event/
Note the caveats near the bottom of that reference, referring to the event when working with images:
It doesn't work consistently nor reliably cross-browser
It doesn't fire correctly in WebKit if the image src is set to the same src as before
It doesn't correctly bubble up the DOM tree
Can cease to fire for images that already live in the browser's cache

javascript to cancel image loading

I am looking for a way to cancel image loading using javascript. I've looked at other questions and hiding them is not enough. Also, the rest of the page must load (window.stop() is out of the question).
The page that is being loaded is not under my control, only one thing is guaranteed - the first <script> on the page is my javascript (lightweight - no jquery).
I have tried setting all img sources to nothing, that did not help since the dom is created after the page is parsed, and all browsers have the same behavior - the img is loaded once it is parsed.

Not possible with modern browsers. Even if you alter the src attribute of image tag with JavaScript browsers still insist on loading the images. I know this from developing the Lazy Load plugin.

The only way I can see to stop images loading is to not have an src attribute present in the image itself, and using a custom data-* attribute to hold the location of the image:
<img data-src="http://path.to/image.png" />
Obviously this doesn't gracefully degrade for those (admittedly rare) JavaScript disabled browsers, and therefore requires a noscript fall-back:
<img data-src="http://path.to/image.png" />
<noscript><img src="http://path.to/image.png" /></noscript>
And couple this with a simple function to load the images when you, or your users, are ready for them:
/* simple demo */
function imagePopulate(within, attr) {
within = within && within.nodeType == 1 ? within : document;
attr = attr || 'data-src';
var images = within.getElementsByTagName('img');
for (var i = 0, len = images.length; i < len; i++) {
if (images[i].parentNode.tagName.toLowerCase() !== 'noscript') {
images[i].src = images[i].getAttribute(attr);
}
}
}
document.getElementById('addImages').onclick = function(){
imagePopulate();
};
JS Fiddle demo.
I can't be sure for all browsers, but this seems to work in Chrome (in that there's no attempt, from looking at the network tab of the developer tools, to load the noscript-contained img).

It can be done with webworkers. See the following example:
https://github.com/NathanWalker/ng2-image-lazy-load.
Stopping a web worker cancels the image loading in browser

Recalling the onload event:
window.onload=function(){
imgs = document.getElementsByTagName('img');
for(i = 0; i < imgs.length(); i++){
imgs[i].src = '#';
}
};

If you want to only cancel the loading of the image , you can use sємsєм's solution
but i do not think it will work by using an window onload event .
You will probably need to provide a button to cancel the image load. Also i suggest, instead of setting the src attribute to "#" , you can remove the src attribute itself using
removeAttribute()
[Make sure you disable the cache while testing]

You need a proxy.
Your script can redirect to another server using something like
location.replace('http://yourserver.com/rewrite/php?url='+escape(this.href));
perhaps you tell us why you want to cancel image loading and whose site you are loading on so we can come up with a better solution
If there is nothing on the page other than images, you could try
document.write('<base href="http://yourserver.com/" />');
which will mess with all non-absolute src and hrefs on the page.
UPDATE Horrible hack but perhaps this almost pseudo code (I am off to bed) will do someting
document.write('<script src="jquery.js"></script><div id="myDiv"></div><!-'+'-');
$(function() {
$.get(location.href,function(data) {
$("#myDiv").html(data.replace(/src=/g,'xsrc='));
});
})

The closest you can get to what you maybe want is to have a quickly loaded placeholder image (ie. low resolution version of your image) and a hidden image (eg. {display:none}) in which the large image gets loaded but not displayed. Then in the onload event for the large image swap the images over (eg. display:block for the large image display:none for the smaller). I also use an array (with their url), to reuse any images that have already been opened.
BTW if you open an image in a new webpage when it gets closed then the image loading will be cancelled. So maybe you can do something similar in a webpage using an iframe to display the image.
To close the iframe and therefore unload the image, remove the frame from the DOM
(another advantage is that browsers spawn another process to deal with iframes, so the page won't lock up while the image loads)

HTML / Javascript wait for graphics to be drawn

I've heard about the onload function which is called after the element is fully loaded.
In the case of graphics or images, does that mean it will wait until the image is displayed in the browser?
<body onload="foo()">...
<img onload="bar();"....
If not, is there a way to get the event when all graphics are drawn and images are displayed on a page?
In my case it´s only one 1600*1200 jpeg image and i draw on it. But the image has to be displayed before i start drawing, even with the onload event i see the drawed lines before the image appear.

Yes body onload will wait until all images (and other content) are loaded/displayed in the browser. The img onload will wait until that specific image has loaded/is displayed

Images have a complete property that's true when they are loaded.
e.g. would test if everything has loaded:
var allImagesLoaded = true;
$("IMG").each(function(){ allImagesLoaded &= $(this).attr("complete"); });
if(allImagesLoaded){ alert("Done!");}
Images raise a load event once they've finished loading

why dont you keep a counter for your images that will decrement by one on each image load.
check if it equal to 0 then call some another function.
in this way you can do the thing you want to when all images are loaded
$(function() {
$('img').one('load',function() {
// fire when image loads decrement the counter
if counter ==0
fireanotherfunction()
});
});
by above code u can attain your purpose

When reading the jQuery ready API documentation here:
While JavaScript provides the load event for executing code when a
page is rendered, this event does not get triggered until all assets
such as images have been completely received.
So onload is launched after everything has been loaded (and displayed).

See the window.load event:
The load event fires at the end of the document loading process. At this point, all of the objects in the document are in the DOM, and all the images and sub-frames have finished loading.
This is exact what you want, I believe.
JQuery's $(document).ready is not what you want:
In cases where code relies on loaded assets (for example, if the dimensions of an image are required), the code should be placed in a handler for the load event [instead of the ready event].
If you're using plain JS, window.load is what you want.
If you are using jQuery, you'll want $(document).load.

try jquery ready function
$(document).ready(function(){
bar();
});
I'm not sure if it works, but it's a try :D

I have the same problem developing a web view for an Android app. The load events (both for window and image element) as well as the complete state of the image element fire too early. My (svg) image has not yet finished drawing and thus calculations on the size go wrong.
The only workaround that I have found is a very short timer (1ms or maybe 10ms). That works for me because I have only one such image to consider. And since I start this timer when the image data has already loaded, this short lapse should be long enough for the device to paint the image.
window.addEventListener('load', function() {
var img = document.getElementById('logo');
window.setTimeout(function(){
var imgRatio = img.naturalWidth / img.naturalHeight;
var renderedWidth = parseInt(window.getComputedStyle(img).width.match(/(\d+)px/));
console.log(renderedWidth, img.complete);
if (renderedWidth < img.naturalWidth) {
img.style.height = (renderedWidth / imgRatio) + 'px';
}
}, 1);
}
Instead of the window load event, the image's load event should also work. But I found it safer to wait for everything, because other elements might affect the drawing of my image.