A Blob object represents a file-like object of immutable, raw data. Blobs represent data that isn't necessarily in a JavaScript-native format. The File interface is based on Blob, inheriting blob functionality and expanding it to support files on the user's system.

To construct a Blob from other non-blob objects and data, use the Blob() constructor. To create a blob that contains a subset of another blob's data, use the slice() method. To obtain a Blob object for a file on the user's file system, see the File documentation.

The APIs accepting Blob objects are also listed on the File documentation.

Note: The slice() method had initially taken length as the second argument to indicate the number of bytes to copy into the new Blob. If you specified values such that start + length exceeded the size of the source Blob, the returned Blob contained data from the start index to the end of the source Blob.

Note: Be aware that the slice() method has vendor prefixes on some browsers and versions: blob.mozSlice() for Firefox 12 and earlier and blob.webkitSlice() in Safari. An old version of the slice() method, without vendor prefixes, had different semantics, and is obsolete. The support for blob.mozSlice() has been dropped with Firefox 30.


Blob(blobParts[, options])
Returns a newly created Blob object whose content consists of the concatenation of the array of values given in parameter.


Blob.isClosed Read only
A boolean value, indicating whether the Blob.close() method has been called on the blob. Closed blobs can not be read.
Blob.size Read only
The size, in bytes, of the data contained in the Blob object.
Blob.type Read only
A string indicating the MIME type of the data contained in the Blob. If the type is unknown, this string is empty.


Closes the blob object, possibly freeing underlying resources.
Blob.slice([start[, end[, contentType]]])
Returns a new Blob object containing the data in the specified range of bytes of the source Blob.


Blob constructor example usage

The Blob() constructor allows one to create blobs from other objects. For example, to construct a blob from string:

var debug = {hello: "world"};
var blob = new Blob([JSON.stringify(debug, null, 2)], {type : 'application/json'});

Before the Blob constructor was available, this could be accomplished through the BlobBuilder API, which is now deprecated:

var builder = new BlobBuilder();
var fileParts = ['<a id="a"><b id="b">hey!</b></a>'];
var myBlob = builder.getBlob('text/xml');

Example for creating a URL to a typed array using a blob

The following code:

var typedArray = GetTheTypedArraySomehow();
var blob = new Blob([typedArray], {type: 'application/octet-binary'}); // pass a useful mime type here
var url = URL.createObjectURL(blob);
// url will be something like: blob:d3958f5c-0777-0845-9dcf-2cb28783acaf
// now you can use the url in any context that regular URLs can be used in, for example img.src, etc.

Example for extracting data from a Blob

The only way to read content from a Blob is to use a FileReader. The following code reads the content of a Blob as a typed array.

var reader = new FileReader();
reader.addEventListener("loadend", function() {
   // reader.result contains the contents of blob as a typed array

By using other methods of FileReader, it is possible to read the contents of a Blob as a string or a data URL.


Specification Status Comment
File API
The definition of 'Blob' in that specification.
Working Draft Initial definition

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 5[1] 4[2] 10 11.10[1] 5.1[1]
slice() 10 webkit
5 moz[3]
10 12 5.1 webkit
Blob() constructor 20 13.0 (13.0) 10 12.10 6
close() and isClosed ? No support[4] ? ? ?
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support ? 13.0 (13.0) ? ? ?
slice() ? ? ? ? ?
Blob() constructor ? ? ? ? ?
close() and isClosed ? No support[4] ? ? ?

[1] A version of slice() taking the length as second argument was implemented in WebKit and Opera 11.10. However, since that syntax differed from Array.slice() and String.slice(), WebKit removed support and added support for the new syntax as Blob.webkitSlice().

[2] A version of slice() taking the length as second argument was implemented in Firefox 4. However, since that syntax differed from Array.slice() and String.slice(), Gecko removed support and added support for the new syntax as mozSlice().

[3] Prior to Gecko 12.0 (Firefox 12.0 / Thunderbird 12.0 / SeaMonkey 2.9), there was a bug that affected the behavior of slice(); it did not work for start and end positions outside the range of signed 64-bit values; it has now been fixed to support unsigned 64-bit values.

[4] See bug 1048325

Gecko notes: availability in privileged code

To use from chrome code, JSM and Bootstrap scope, you have to import it like this:


Blob is available in Worker scopes.

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

API Files NeedsMobileBrowserCompatibility Reference WebAPI