Refresh image with a new one at the same url
As an alternative to...
newImage.src = "http://localhost/image.jpg?" + new Date().getTime();
...it seems that...
newImage.src = "http://localhost/image.jpg#" + new Date().getTime();
...is sufficient to fool the browser cache without bypassing any upstream caches, assuming you returned the correct Cache-Control headers. Although you can use...
Cache-Control: no-cache, must-revalidate
...you lose the benefits of the If-Modified-Since or If-None-Match headers, so something like...
Cache-Control: max-age=0, must-revalidate
...should prevent the browser from re-downloading the entire image if it hasn't actually changed. Tested and working on IE, Firefox, and Chrome. Annoyingly it fails on Safari unless you use...
Cache-Control: no-store
...although this still may be preferable to filling upstream caches with hundreds of identical images, particularly when they're running on your own server. ;-)
Update (2014-09-28): Nowadays it looks like Cache-Control: no-store is needed for Chrome as well.
Try adding a cachebreaker at the end of the url:
newImage.src = "http://localhost/image.jpg?" + new Date().getTime();
This will append the current timestamp automatically when you are creating the image, and it will make the browser look again for the image instead of retrieving the one in the cache.
I've seen a lot of variation in answers for how to do this, so I thought I'd summarize them here (plus add a 4th method of my own invention):
(1) Add a unique cache-busting query parameter to the URL, such as:
newImage.src = "image.jpg?t=" + new Date().getTime();
Pros: 100% reliable, quick & easy to understand and implement.
Cons: Bypasses caching altogether, meaning unnecessary delays and bandwidth use whenever the image doesn't change between views. Will potentially fill browser cache (and any intermediate caches) with many, many copies of exactly the same image! Also, requires modifying image URL.
When to use: Use when image is constantly changing, such as for a live webcam feed. If you use this method, make sure to serve the images themselves with Cache-control: no-cache HTTP headers!!! (Often this can be set up using a .htaccess file). Otherwise you'll be progressively filling caches up with old versions of the image!
(2) Add query parameter to the URL that changes only when the file does, e.g.:
echo '<img src="image.jpg?m=' . filemtime('image.jpg') . '">';
(That's PHP server-side code, but the important point here is just that a ?m=[file last-modified time] querystring is appended to the filename).
Pros: 100% reliable, quick & easy to understand and implement, and preserves caching advantages perfectly.
Cons: Requires modifying the image URL. Also, a little more work for the server - it has to get access to the file-last-modified time. Also, requires server-side information, so not suitable for a purely client-side-only solution to check for a refreshed image.
When to use: When you want to cache images, but may need to update them at the server end from time to time without changing the filename itself. AND when you can easily ensure that the correct querystring is added to every image instance in your HTML.
(3) Serve your images with the header Cache-control: max-age=0, must-revalidate, and add a unique memcache-busting fragment identifier to the URL, such as:
newImage.src = "image.jpg#" + new Date().getTime();
The idea here is that the cache-control header puts images in the browser cache, but immediately markes them stale, so that and every time they are re-displayed the browser must check with the server to see if they've changed. This ensures that the browser's HTTP cache always returns the latest copy of the image. However, browsers will often re-use an in-memory copy of an image if they have one, and not even check their HTTP cache in that case. To prevent this, a fragment identifier is used: Comparison of in-memory image src's includes the fragment identifier, but it gets stripped of before querying the HTTP cache. (So, e.g., image.jpg#A and image.jpg#B might both be displayed from the image.jpg entry in the browser's HTTP cache, but image.jpg#B would never be displayed using in-memory retained image data from when image.jpg#A was last displayed).
Pros: Makes proper use of HTTP caching mechanisms, and uses cached images if they haven't changed. Works for servers that choke on a querystring added to a static image URL (since servers never see fragment identifiers - they're for the browsers' own use only).
Cons: Relies on somewhat dubious (or at least poorly documented) behaviour of browsers, in regard to images with fragment identifiers in their URLs (However, I've tested this successfully in FF27, Chrome33, and IE11). Does still send a revalidation request to the server for every image view, which may be overkill if images only change rarely and/or latency is a big issue (since you need to wait for the revalidation response even when the cached image is still good). Requires modifying image URLs.
When to use: Use when images may change frequently, or need to be refreshed intermittently by the client without server-side script involvement, but where you still want the advantage of caching. For example, polling a live webcam that updates an image irregularly every few minutes. Alternatively, use instead of (1) or (2) if your server doesn't allow querystrings on static image URLs.
[EDIT 2021: No longer works on recent Chrome & Edge: The internal memcache in those browsers now ignores fragment identifiers (maybe since the switch to the Blink engine?). But see method (4) below, it's now MUCH easier on those two browsers specifically, so consider combining this method with a simplified version of (4) to cover those two browsers].
(4) Forcibly refresh a particular image using Javascript, by first loading it into a hidden <iframe> and then calling location.reload(true) on the iframe's contentWindow.
The steps are:
Load the image to be refreshed into a hidden iframe. [EDIT 2021: For Chrome and Edge, load a HTML page with an
<img>tag, not the raw image file]. This is just a setup step - it can be done long in advance the actual refresh, if desired. It doesn't even matter if the image fails to load at this stage![EDIT 2021: This step is now unnecessary in recent Chrome and Edge]. Once that's done, blank out all copies of that image on your page(s) or anywhere in any DOM nodes (even off-page ones stored in javascript variables). This is necessary because the browser may otherwise display the image from a stale in-memory copy (IE11 especially does this): You need to ensure all in-memory copies are cleared, before refreshing the HTTP cache. If other javascript code is running asynchronously, you may also need to prevent that code from creating new copies of the to-be-refreshed image in the meantime.
Call
iframe.contentWindow.location.reload(true). Thetrueforces a cache bypass, reloading directly from the server and overwriting the existing cached copy.[EDIT 2021: This step is now unnecessary in recent Chrome and Edge - on those browsers, existing images will just automatically update themselves after the previous step!] Once it's finished re-loading, restore the blanked images. They should now display the fresh version from the server!
For same-domain images, you can load the image into the iframe directly. [EDIT 2021: Not on Chrome, Edge]. For cross-domain images, you have to instead load a HTML page from your domain that contains the image in an <img> tag, otherwise you'll get an "Access Denied" error when trying to call iframe.contentWindow.reload(...). [Do this for Chrome & Edge also].
Pros: Works just like the image.reload() function you wish the DOM had! Allows images to by cached normally (even with in-the-future expiry dates if you want them, thus avoiding frequent revalidation). Allows you to refresh a particular image without altering the URLs for that image on the current page, or on any other pages, using only client-side code.
Cons: Relies on Javascript. Not 100% guaranteed to work properly in every browser (I've tested this successfully in FF27, Chrome33, and IE11 though). Very complicated relative to the other methods. [EDIT 2021: Unless you only need recent Chrome & Edge support, in which case it's very much simpler].
When to use: When you have a collection of basically static images that you'd like cached, but you still need to be able to update them occasionally and get immediate visual feedback that the update took place. (Especially when just refreshing the whole browser page wouldn't work, as in some web apps built on AJAX for example). And when methods (1)-(3) aren't feasible because (for whatever reason) you can't change all the URLs that might potentially display the image you need to have updated. (Note that using those 3 methods the image will be refreshed, but if another page then tries to displays that image without the appropriate querystring or fragment identifier, it may show an older version instead).
The details of implementing this in a fairy robust and flexible manner are given below:
Let's assume your website contains a blank 1x1 pixel .gif at the URL path /img/1x1blank.gif, and also has the following one-line PHP script (only required for applying forced refresh to cross-domain images, and can be rewritten in any server-side scripting language, of course) at the URL path /echoimg.php:
<img src="<?=htmlspecialchars(@$_GET['src'],ENT_COMPAT|ENT_HTML5,'UTF-8')?>">
Then, here's a realistic implementation of how you might do all this in Javascript. It looks a bit complicated, but there's a lot of comments, and the important function is just forceImgReload() - the first two just blank and un-blank images, and should be designed to work efficiently with your own HTML, so code them as works best for you; much of the complications in them may be unnecessary for your website:
// This function should blank all images that have a matching src, by changing their src property to /img/1x1blank.gif.
// ##### You should code the actual contents of this function according to your page design, and what images there are on them!!! #####
// Optionally it may return an array (or other collection or data structure) of those images affected.
// This can be used by imgReloadRestore() to restore them later, if that's an efficient way of doing it (otherwise, you don't need to return anything).
// NOTE that the src argument here is just passed on from forceImgReload(), and MAY be a relative URI;
// However, be aware that if you're reading the src property of an <img> DOM object, you'll always get back a fully-qualified URI,
// even if the src attribute was a relative one in the original HTML. So watch out if trying to compare the two!
// NOTE that if your page design makes it more efficient to obtain (say) an image id or list of ids (of identical images) *first*, and only then get the image src,
// you can pass this id or list data to forceImgReload() along with (or instead of) a src argument: just add an extra or replacement parameter for this information to
// this function, to imgReloadRestore(), to forceImgReload(), and to the anonymous function returned by forceImgReload() (and make it overwrite the earlier parameter variable from forceImgReload() if truthy), as appropriate.
function imgReloadBlank(src)
{
// ##### Everything here is provisional on the way the pages are designed, and what images they contain; what follows is for example purposes only!
// ##### For really simple pages containing just a single image that's always the one being refreshed, this function could be as simple as just the one line:
// ##### document.getElementById("myImage").src = "/img/1x1blank.gif";
var blankList = [],
fullSrc = /* Fully qualified (absolute) src - i.e. prepend protocol, server/domain, and path if not present in src */,
imgs, img, i;
for each (/* window accessible from this one, i.e. this window, and child frames/iframes, the parent window, anything opened via window.open(), and anything recursively reachable from there */)
{
// get list of matching images:
imgs = theWindow.document.body.getElementsByTagName("img");
for (i = imgs.length; i--;) if ((img = imgs[i]).src===fullSrc) // could instead use body.querySelectorAll(), to check both tag name and src attribute, which would probably be more efficient, where supported
{
img.src = "/img/1x1blank.gif"; // blank them
blankList.push(img); // optionally, save list of blanked images to make restoring easy later on
}
}
for each (/* img DOM node held only by javascript, for example in any image-caching script */) if (img.src===fullSrc)
{
img.src = "/img/1x1blank.gif"; // do the same as for on-page images!
blankList.push(img);
}
// ##### If necessary, do something here that tells all accessible windows not to create any *new* images with src===fullSrc, until further notice,
// ##### (or perhaps to create them initially blank instead and add them to blankList).
// ##### For example, you might have (say) a global object window.top.blankedSrces as a propery of your topmost window, initially set = {}. Then you could do:
// #####
// ##### var bs = window.top.blankedSrces;
// ##### if (bs.hasOwnProperty(src)) bs[src]++; else bs[src] = 1;
// #####
// ##### And before creating a new image using javascript, you'd first ensure that (blankedSrces.hasOwnProperty(src)) was false...
// ##### Note that incrementing a counter here rather than just setting a flag allows for the possibility that multiple forced-reloads of the same image are underway at once, or are overlapping.
return blankList; // optional - only if using blankList for restoring back the blanked images! This just gets passed in to imgReloadRestore(), it isn't used otherwise.
}
// This function restores all blanked images, that were blanked out by imgReloadBlank(src) for the matching src argument.
// ##### You should code the actual contents of this function according to your page design, and what images there are on them, as well as how/if images are dimensioned, etc!!! #####
function imgReloadRestore(src,blankList,imgDim,loadError);
{
// ##### Everything here is provisional on the way the pages are designed, and what images they contain; what follows is for example purposes only!
// ##### For really simple pages containing just a single image that's always the one being refreshed, this function could be as simple as just the one line:
// ##### document.getElementById("myImage").src = src;
// ##### if in imgReloadBlank() you did something to tell all accessible windows not to create any *new* images with src===fullSrc until further notice, retract that setting now!
// ##### For example, if you used the global object window.top.blankedSrces as described there, then you could do:
// #####
// ##### var bs = window.top.blankedSrces;
// ##### if (bs.hasOwnProperty(src)&&--bs[src]) return; else delete bs[src]; // return here means don't restore until ALL forced reloads complete.
var i, img, width = imgDim&&imgDim[0], height = imgDim&&imgDim[1];
if (width) width += "px";
if (height) height += "px";
if (loadError) {/* If you want, do something about an image that couldn't load, e.g: src = "/img/brokenImg.jpg"; or alert("Couldn't refresh image from server!"); */}
// If you saved & returned blankList in imgReloadBlank(), you can just use this to restore:
for (i = blankList.length; i--;)
{
(img = blankList[i]).src = src;
if (width) img.style.width = width;
if (height) img.style.height = height;
}
}
// Force an image to be reloaded from the server, bypassing/refreshing the cache.
// due to limitations of the browser API, this actually requires TWO load attempts - an initial load into a hidden iframe, and then a call to iframe.contentWindow.location.reload(true);
// If image is from a different domain (i.e. cross-domain restrictions are in effect, you must set isCrossDomain = true, or the script will crash!
// imgDim is a 2-element array containing the image x and y dimensions, or it may be omitted or null; it can be used to set a new image size at the same time the image is updated, if applicable.
// if "twostage" is true, the first load will occur immediately, and the return value will be a function
// that takes a boolean parameter (true to proceed with the 2nd load (including the blank-and-reload procedure), false to cancel) and an optional updated imgDim.
// This allows you to do the first load early... for example during an upload (to the server) of the image you want to (then) refresh.
function forceImgReload(src, isCrossDomain, imgDim, twostage)
{
var blankList, step = 0, // step: 0 - started initial load, 1 - wait before proceeding (twostage mode only), 2 - started forced reload, 3 - cancelled
iframe = window.document.createElement("iframe"), // Hidden iframe, in which to perform the load+reload.
loadCallback = function(e) // Callback function, called after iframe load+reload completes (or fails).
{ // Will be called TWICE unless twostage-mode process is cancelled. (Once after load, once after reload).
if (!step) // initial load just completed. Note that it doesn't actually matter if this load succeeded or not!
{
if (twostage) step = 1; // wait for twostage-mode proceed or cancel; don't do anything else just yet
else { step = 2; blankList = imgReloadBlank(src); iframe.contentWindow.location.reload(true); } // initiate forced-reload
}
else if (step===2) // forced re-load is done
{
imgReloadRestore(src,blankList,imgDim,(e||window.event).type==="error"); // last parameter checks whether loadCallback was called from the "load" or the "error" event.
if (iframe.parentNode) iframe.parentNode.removeChild(iframe);
}
}
iframe.style.display = "none";
window.parent.document.body.appendChild(iframe); // NOTE: if this is done AFTER setting src, Firefox MAY fail to fire the load event!
iframe.addEventListener("load",loadCallback,false);
iframe.addEventListener("error",loadCallback,false);
iframe.src = (isCrossDomain ? "/echoimg.php?src="+encodeURIComponent(src) : src); // If src is cross-domain, script will crash unless we embed the image in a same-domain html page (using server-side script)!!!
return (twostage
? function(proceed,dim)
{
if (!twostage) return;
twostage = false;
if (proceed)
{
imgDim = (dim||imgDim); // overwrite imgDim passed in to forceImgReload() - just in case you know the correct img dimensions now, but didn't when forceImgReload() was called.
if (step===1) { step = 2; blankList = imgReloadBlank(src); iframe.contentWindow.location.reload(true); }
}
else
{
step = 3;
if (iframe.contentWindow.stop) iframe.contentWindow.stop();
if (iframe.parentNode) iframe.parentNode.removeChild(iframe);
}
}
: null);
}
Then, to force a refresh of an image located on the same domain as your page, you can just do:
forceImgReload("myimage.jpg");
To refresh an image from somewhere else (cross-domain):
forceImgReload("http://someother.server.com/someimage.jpg", true);
A more advanced application might be to reload an image after uploading a new version to your server, preparing the initial stage of the reload process simultaneous with the upload, to minimize the visible reload delay to the user. If you're doing the upload via AJAX, and the server is returning a very simple JSON array [success, width, height] then your code might look something like this:
// fileForm is a reference to the form that has a the <input typ="file"> on it, for uploading.
// serverURL is the url at which the uploaded image will be accessible from, once uploaded.
// The response from uploadImageToServer.php is a JSON array [success, width, height]. (A boolean and two ints).
function uploadAndRefreshCache(fileForm, serverURL)
{
var xhr = new XMLHttpRequest(),
proceedWithImageRefresh = forceImgReload(serverURL, false, null, true);
xhr.addEventListener("load", function(){ var arr = JSON.parse(xhr.responseText); if (!(arr&&arr[0])) { proceedWithImageRefresh(false); doSomethingOnUploadFailure(...); } else { proceedWithImageRefresh(true,[arr[1],ar[2]]); doSomethingOnUploadSuccess(...); }});
xhr.addEventListener("error", function(){ proceedWithImageRefresh(false); doSomethingOnUploadError(...); });
xhr.addEventListener("abort", function(){ proceedWithImageRefresh(false); doSomethingOnUploadAborted(...); });
// add additional event listener(s) to track upload progress for graphical progress bar, etc...
xhr.open("post","uploadImageToServer.php");
xhr.send(new FormData(fileForm));
}
A final note: Although this topic is about images, it potentially applies to other kinds of files or resources also. For example, preventing the use of stale script or css files, or perhaps even refreshing updated PDF documents (using (4) only if set up to open in-browser). Method (4) might require some changes to the above javascript, in these cases.