207

I want to know when an image has finished loading. Is there a way to do it with a callback?

If not, is there a way to do it at all?

Improve this question
1

12 Answers 12

Reset to default
241

.complete + callback

This is a standards compliant method without extra dependencies, and waits no longer than necessary:

var img = document.querySelector('img')

function loaded() {
  alert('loaded')
}

if (img.complete) {
  loaded()
} else {
  img.addEventListener('load', loaded)
  img.addEventListener('error', function() {
      alert('error')
  })
}

Source: http://www.html5rocks.com/en/tutorials/es6/promises/

Improve this answer
Sign up to request clarification or add additional context in comments.

8 Comments

Can this go wrong if the image completes between the img.complete call and the addEventListener call?
@ThomasAhle I'm not an expert, but that seems possible. Let's see if anyone has a solution.
I guess it would only be an issue for parallel code, which most javascript probably isn't..
@ThomasAhle It can't, because the browser will only fire the load event when the event queue is spun. That said, the load event listener must be in an else clause, as img.complete can become true before the load event is fired, hence if it weren't you could potentially have loaded called twice (note this is relatively likely to change such that img.complete only becomes true when the event fires).
I think we can also add img.decode() to the first condition; if(img.complete || img.decode())
|
94

Image.onload() will often work.

To use it, you'll need to be sure to bind the event handler before you set the src attribute.

Related Links:

Example Usage:

    window.onload = function () {

        var logo = document.getElementById('sologo');

        logo.onload = function () {
            alert ("The image has loaded!");		
        };

        setTimeout(function(){
            logo.src = 'https://edmullen.net/test/rc.jpg';         
        }, 5000);
    };
 <html>
    <head>
    <title>Image onload()</title>
    </head>
    <body>

    <img src="#" alt="This image is going to load" id="sologo"/>

    <script type="text/javascript">

    </script>
    </body>
    </html>

Improve this answer

13 Comments

FYI: According ot the W3C spec, onload is not a valid event for IMG elements. Obviously browsers do support it, but if you care about the spec and are not 100% sure all of the browsers you want to target support this, you may want to rethink it or at least keep it in mind.
why does waiting 5 seconds to set the source seem like a bad idea?
@quemeful that's why it's called "an example".
@JasonBunting What are you looking at that makes you think the load event isn't valid? html.spec.whatwg.org/multipage/webappapis.html#handler-onload is the definition of the onload event handler.
@gsnedders - you realize, I'd assume, that when I wrote that comment, I was referring to the extant standard, not the one that you pointed to, which is 4.5 years newer. Would that you had asked back then, I could have pointed to it, perhaps. Such is the nature of the web, right? Things get old or are moved or are modified, etc.
|
21

You can use the .complete property of the Javascript image class.

I have an application where I store a number of Image objects in an array, that will be dynamically added to the screen, and as they're loading I write updates to another div on the page. Here's a code snippet:

var gAllImages = [];

function makeThumbDivs(thumbnailsBegin, thumbnailsEnd)
{
    gAllImages = [];

    for (var i = thumbnailsBegin; i < thumbnailsEnd; i++) 
    {
        var theImage = new Image();
        theImage.src = "thumbs/" + getFilename(globals.gAllPageGUIDs[i]);
        gAllImages.push(theImage);

        setTimeout('checkForAllImagesLoaded()', 5);
        window.status="Creating thumbnail "+(i+1)+" of " + thumbnailsEnd;

        // make a new div containing that image
        makeASingleThumbDiv(globals.gAllPageGUIDs[i]);
    }
}

function checkForAllImagesLoaded()
{
    for (var i = 0; i < gAllImages.length; i++) {
        if (!gAllImages[i].complete) {
            var percentage = i * 100.0 / (gAllImages.length);
            percentage = percentage.toFixed(0).toString() + ' %';

            userMessagesController.setMessage("loading... " + percentage);
            setTimeout('checkForAllImagesLoaded()', 20);
            return;
        }
    }

    userMessagesController.setMessage(globals.defaultTitle);
}
Improve this answer

2 Comments

I've used similar code in my work before. This works well in all browsers.
The problem with checking complete is that starting with IE9 the OnLoad events is fired before all images are loaded and its hard to find a trigger for the checking function now.
20

Life is too short for jquery.

function waitForImageToLoad(imageElement){
  return new Promise(resolve=>{imageElement.onload = resolve})
}

var myImage = document.getElementById('myImage');
var newImageSrc = "https://pmchollywoodlife.files.wordpress.com/2011/12/justin-bieber-bio-photo1.jpg?w=620"

myImage.src = newImageSrc;
waitForImageToLoad(myImage).then(()=>{
  // Image have loaded.
  console.log('Loaded lol')
});
<img id="myImage" src="">

Improve this answer

3 Comments

This is a great answer but I don't think you even need that much code. You're just making it look confusing by adding the src with JS.
Time is a huge constraint in programming . From my exprience , writing readable (while somtime long) code gives a huge time benefit.
Why would you store the src into a new variable only to consume it on the next line? If brevity is your goal, that's an anti-pattern. myImage.src = https://pmchollywoodlife.files.wordpress.com/2011/12/justin-bieber-bio-photo1.jpg?w=620 does the trick.
12

You could use the load()-event in jQuery but it won't always fire if the image is loaded from the browser cache. This plugin https://github.com/peol/jquery.imgloaded/raw/master/ahpi.imgload.js can be used to remedy that problem.

Improve this answer

Comments

6

If the goal is to style the image after the browser has rendered it, you should:

const img = new Image();
img.src = 'path/to/img.jpg';

img.decode().then(() => {
  /* set styles */
  /* add img to DOM */ 
});

Because the browser first loads the compressed version of image, then decodes it, and finally paints it. Since there is no event for paint you should run your logic after browser has decoded the img tag.

Improve this answer

Comments

5

Here is jQuery equivalent:

var $img = $('img');

if ($img.length > 0 && !$img.get(0).complete) {
   $img.on('load', triggerAction);
}

function triggerAction() {
   alert('img has been loaded');
}
Improve this answer

Comments

5

Not suitable for 2008 when the question was asked, but these days this works well for me:

async function newImageSrc(src) {
  // Get a reference to the image in whatever way suits.
  let image = document.getElementById('image-id');

  // Update the source.
  image.src = src;

  // Wait for it to load.
  await new Promise((resolve) => { image.onload = resolve; });

  // Done!
  console.log('image loaded! do something...');
}
Improve this answer

1 Comment

I assume you meant image.src = src;
2

Sorry, I can`t resist to simplify Ciro Santilli's answer from 2014.
This is code from a real-world webpage:

if (img.complete)  loaded();
else  img.onload = loaded;

or just:

img.complete ? loaded() : img.onload = loaded;

you may also consider the error case:

img.complete ? loaded() : img.onload = img.onerror = loaded;
Improve this answer

Comments

0

these functions will solve the problem, you need to implement the DrawThumbnails function and have a global variable to store the images. I love to get this to work with a class object that has the ThumbnailImageArray as a member variable, but am struggling!

called as in addThumbnailImages(10);

var ThumbnailImageArray = [];

function addThumbnailImages(MaxNumberOfImages)
{
    var imgs = [];

    for (var i=1; i<MaxNumberOfImages; i++)
    {
        imgs.push(i+".jpeg");
    }

    preloadimages(imgs).done(function (images){
            var c=0;

            for(var i=0; i<images.length; i++)
            {
                if(images[i].width >0) 
                {
                    if(c != i)
                        images[c] = images[i];
                    c++;
                }
            }

            images.length = c;

            DrawThumbnails();
        });
}



function preloadimages(arr)
{
    var loadedimages=0
    var postaction=function(){}
    var arr=(typeof arr!="object")? [arr] : arr

    function imageloadpost()
    {
        loadedimages++;
        if (loadedimages==arr.length)
        {
            postaction(ThumbnailImageArray); //call postaction and pass in newimages array as parameter
        }
    };

    for (var i=0; i<arr.length; i++)
    {
        ThumbnailImageArray[i]=new Image();
        ThumbnailImageArray[i].src=arr[i];
        ThumbnailImageArray[i].onload=function(){ imageloadpost();};
        ThumbnailImageArray[i].onerror=function(){ imageloadpost();};
    }
    //return blank object with done() method    
    //remember user defined callback functions to be called when images load
    return  { done:function(f){ postaction=f || postaction } };
}
Improve this answer

1 Comment

Most of this code is irrelevant, including your entire addThumbnailImages function. Pare it down to the relevant code and I'll remove the -1.
0

This worked for me:

// Usage
let img = await image_on_load(parent_element_to_put_img, image_url);
img.hidden = true;

// Functions
async function image_on_load(parent, url) {
    let img = element(parent, 'img');
    img.src = url;
    await new Promise((resolve, reject) => {
        element_on(img, 'load', () => {
            resolve();
        });
        element_on(img, 'error', () => {
            reject();
        });
    });
    return img;
}
function element(parent, tag_name, text = '') {
    let result = document.createElement(tag_name);
    element_child_append(parent, result);
    element_html_inner(result, text);
    return result;
}
function element_child_append(parent, result) {
    parent.appendChild(result);
}
function element_html_inner(result, text) {
    result.innerHTML = text;
}
function element_on(e, event, on_event) {
    e.addEventListener(event, async () => {
        await on_event();
    });
}
Improve this answer

Comments

-3

If you are using React.js, you could do this:

render() {

// ...

<img 
onLoad={() => this.onImgLoad({ item })}
onError={() => this.onImgLoad({ item })}

src={item.src} key={item.key}
ref={item.key} />

// ... }

Where:

  • - onLoad (...) now will called with something like this: { src: "https://......png", key:"1" } you can use this as "key" to know which images is loaded correctly and which not.
  • - onError(...) it is the same but for errors.
  • - the object "item" is something like this { key:"..", src:".."} you can use to store the images' URL and key in order to use in a list of images.

    • Improve this answer

    Comments

    Your Answer

    By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.