Html5's File API - BLOB usages?

The length of a blob: URL is always below a reasonable limit.

Data URLs can be arbitrary large. Consequently, when a data URL is too long, some browsers (IE, cough) will not display the image any more. So, if you want to display very large files, using blob: (or filesystem: URLs) might make more sense than data-URLs.

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Also, you can directly recover data from a blob: URL ^{(provided that the blob has not been revoked yet, e.g. because the document was unloaded, and the same origin policy is not violated)} using XMLHttpRequest. For example, the following code gets the content of a blob URL as text:

var blobUrl = URL.createObjectURL(new Blob(['Test'], {type: 'text/plain'}));
var x = new XMLHttpRequest();
// set x.responseType = 'blob' if you want to get a Blob object:
// x.responseType = 'blob';
x.onload = function() {
    alert(x.responseText);
};
x.open('get', blobUrl);
x.send();

If you want to submit the contents of a File to a server using XMLHttpRequest, it doesn't really make sense to use a blob: or data: URL. Just submit the File object directly using the FormData object. If you lost the original File reference, and you only have a blob: URL, then you can use the previous snippet to get a Blob object again for use in FormData.

Given a data:-URL, it is far from easy to recover the original data. Firefox and Opera 12- allow use of a data:-URL in XMLHttpRequest. Chrome, Internet Explorer, Safari and Opera 15+ refuse to load a data-URL via XMLHttpRequest. So, with respect to recovering data, blob: URLs are also superior over data:-URLs.

If you want to display the result of a File in a different frame on the same origin, definitely use a blob: URL. If you want to manipulate data contained in a Blob in a differerent frame (possibly on a different origin), do not use blob or data URLs, send the data directly using postMessage.

blob:-URLs are generally better than data:-URLs for representing (binary) data. For small data (max 20kb), data: URLs might be a better choice because of the higher range of supported browsers: Compare Can I use Blob URLs with Can I use Data URIs ^{(though if you're writing a complex HTML5 application, odds are that you're not going to support IE9-)}.

Here's the main differences in how you can use the two types of URLs:

Data URLs:

Pros:

• you can get data out of them very easily
• you can send them to another user or across HTTP, and the data's still there
• It doesn't matter where or how they were created, if the data is valid, you'll see the content in any browser, on any OS, anywhere

Cons:

• Data URLs are often prohibitively long, so that IE might not be able to handle them and it can get annoying to handle in any browser
• They are less efficient than BLOB URLs (you have to read the file to create it, you don't with BLOBs, etc)

BLOB URLs:

Pros:

• They are much shorter than Data URLs, making them far more manageable
• you can access their data, but since the URL is only an opaque reference to the data, the data must be accessed using a FileReader and the data cannot be extracted directly from the URL, as in Data URLs
• because they have a reasonable length, they are easier to deal with and have better IE support

Cons:

• The data isn't accessible in the URL itself (the URL is an opaque reference) and it isn't stored in the cloud
• Because of con #1, you can't send the URL to the server/a different user, as they will be unable to access the data. So the URL's only for you.
• You also can't access the data from the BLOB URL in a different browser (even on the same machine)
• Also, you are unable to access a BLOB URL from a different origin, even on the same browser

This list makes it seem like data URLs are an obvious advantage, but BLOB urls are faster to create, and, unless you need to send the url to other users or to the server, you should use them because they are faster, easier to use, more manageable, and better for IE. But if you do need to send a url to the server or to another user, I would recommend somehow transmitting the blob directly using XHR2. Data urls aren't that great.