Download Image Javascript ##TOP##

The entire bitmap is loaded regardless of the sizes specified in the constructor. The size specified in the constructor is reflected through the properties HTMLImageElement.width and HTMLImageElement.height of the resulting instance. The intrinsic width and height of the image in CSS pixels are reflected through the properties HTMLImageElement.naturalWidth and HTMLImageElement.naturalHeight. If no size is specified in the constructor both pairs of properties have the same values.

\n The entire bitmap is loaded regardless of the sizes specified in the constructor. The\n size specified in the constructor is reflected through the properties\n HTMLImageElement.width and HTMLImageElement.height of the\n resulting instance. The intrinsic width and height of the image in CSS pixels are\n reflected through the properties HTMLImageElement.naturalWidth and\n HTMLImageElement.naturalHeight. If no size is specified in the\n constructor both pairs of properties have the same values.\n

Download Image Javascript

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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!

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.

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.

Load the image to be refreshed into a hidden iframe. [EDIT 2021: For Chrome and Edge, load a HTML page with an 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.

[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 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.

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).

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:

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:

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:

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.

By providing a date in the past, it won't be cached by the browser. Cache-Control was added in HTTP/1.1 and the must-revalidate tag indicates that proxies should never serve up an old image even under extenuating circumstances, and the Pragma: no-cache isn't really necessary for current modern browsers/caches but may help with some crufty broken old implementations.

What I ended up doing was having the server map any request for an image at that directory to the source that I was trying to update. I then had my timer append a number onto the end of the name so the DOM would see it as a new image and load it.

I had this same issue using the Unsplash random image feature. The idea of adding a dummy query string to the end of the URL is correct, but in this instance a completely random parameter doesn't work (I tried it). I can imagine it's the same for some other services too, but for unsplash the parameter needs to be sig, so your image URL would be, for example, =RANDOM where random is a random string that will NOT be the same when you update it. I used Math.random()*100 but date is suitable too.

Heavily based on Doin's #4 code, the below example simplifies that code a great bit utilising document.write instead of src in the iframe to support CORS. Also only focuses on busting the browser cache, not reloading every image on the page.

I improved the script from AlexMA for showing my webcam on a web page wich periodically uploads a new image with the same name. I had issues that sometimes the image was flickering because of a broken image or not complete (up)loaded image. To prevent flickering I check the natural height of the image because the size of my webcam image did not change. Only if the loaded image height fits the original image height the full image will be shown on page. 2351a5e196