What is a blob URL and why it is used?

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Blob URLs (ref W3C, official name) or Object-URLs (ref. MDN and method name) are used with a Blob or a File object.

src="blob:https://crap.crap" I opened the blob url that was in src of video it gave a error and i can't open but was working with the src tag how it is possible?

Blob URLs can only be generated internally by the browser. URL.createObjectURL() will create a special reference to the Blob or File object which later can be released using URL.revokeObjectURL(). These URLs can only be used locally in the single instance of the browser and in the same session (ie. the life of the page/document).

What is blob url?
Why it is used?

Blob URL/Object URL is a pseudo protocol to allow Blob and File objects to be used as URL source for things like images, download links for binary data and so forth.

For example, you can not hand an Image object raw byte-data as it would not know what to do with it. It requires for example images (which are binary data) to be loaded via URLs. This applies to anything that require an URL as source. Instead of uploading the binary data, then serve it back via an URL it is better to use an extra local step to be able to access the data directly without going via a server.

It is also a better alternative to Data-URI which are strings encoded as Base-64. The problem with Data-URI is that each char takes two bytes in JavaScript. On top of that a 33% is added due to the Base-64 encoding. Blobs are pure binary byte-arrays which does not have any significant overhead as Data-URI does, which makes them faster and smaller to handle.

Can i make my own blob url on a server?

No, Blob URLs/Object URLs can only be made internally in the browser. You can make Blobs and get File object via the File Reader API, although BLOB just means Binary Large OBject and is stored as byte-arrays. A client can request the data to be sent as either ArrayBuffer or as a Blob. The server should send the data as pure binary data. Databases often uses Blob to describe binary objects as well, and in essence we are talking basically about byte-arrays.

if you have then Additional detail

You need to encapsulate the binary data as a BLOB object, then use URL.createObjectURL() to generate a local URL for it:

var blob = new Blob([arrayBufferWithPNG], {type: "image/png"}),
    url = URL.createObjectURL(blob),
    img = new Image();

img.onload = function() {
    URL.revokeObjectURL(this.src);     // clean-up memory
    document.body.appendChild(this);   // add image to DOM
}

img.src = url;                         // can now "stream" the bytes

Note that URL may be prefixed in webkit-browsers, so use:

var url = (URL || webkitURL).createObjectURL(...);