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---
title: Blob
slug: Web/API/Blob
tags:
- API
- Files
- NeedsMobileBrowserCompatibility
- NeedsTranslation
- Reference
- TopicStub
- WebAPI
translation_of: Web/API/Blob
---
<div>{{APIRef("File API")}}</div>
<p>A <code>Blob</code> object represents a file-like object of immutable, raw data. Blobs represent data that isn't necessarily in a JavaScript-native format. The {{domxref("File")}} interface is based on <code>Blob</code>, inheriting blob functionality and expanding it to support files on the user's system.</p>
<p>To construct a <code>Blob</code> from other non-blob objects and data, use the {{domxref("Blob.Blob", "Blob()")}} constructor. To create a blob that contains a subset of another blob's data, use the {{domxref("Blob.slice()", "slice()")}} method. To obtain a <code>Blob</code> object for a file on the user's file system, see the {{domxref("File")}} documentation.</p>
<p>The APIs accepting <code>Blob</code> objects are also listed on the {{domxref("File")}} documentation.</p>
<div class="note">
<p><strong>Note:</strong> The <code>slice()</code> method had initially taken <code>length</code> as the second argument to indicate the number of bytes to copy into the new <code>Blob</code>. If you specified values such that <code>start + length</code> exceeded the size of the source <code>Blob</code>, the returned <code>Blob</code> contained data from the start index to the end of the source <code>Blob</code>.</p>
</div>
<div class="note"><strong>Note:</strong> Be aware that the <code>slice()</code> method has vendor prefixes on some browsers and versions: <code>blob.mozSlice()</code> for Firefox 12 and earlier and <code>blob.webkitSlice()</code> in Safari. An old version of the <code>slice()</code> method, without vendor prefixes, had different semantics, and is obsolete. The support for <code>blob.mozSlice()</code> has been dropped with Firefox 30.</div>
<h2 id="Constructor">Constructor</h2>
<dl>
<dt>{{domxref("Blob.Blob", "Blob(blobParts[, options])")}}</dt>
<dd>Returns a newly created <code>Blob</code> object whose content consists of the concatenation of the array of values given in parameter.</dd>
</dl>
<h2 id="Properties">Properties</h2>
<dl>
<dt>{{domxref("Blob.size")}} {{readonlyinline}}</dt>
<dd>The size, in bytes, of the data contained in the <code>Blob</code> object.</dd>
<dt>{{domxref("Blob.type")}} {{readonlyinline}}</dt>
<dd>A string indicating the MIME type of the data contained in the <code>Blob</code>. If the type is unknown, this string is empty.</dd>
</dl>
<h2 id="Methods">Methods</h2>
<dl>
<dt>{{domxref("Blob.slice()", "Blob.slice([start[, end[, contentType]]])")}}</dt>
<dd>Returns a new <code>Blob</code> object containing the data in the specified range of bytes of the source <code>Blob</code>.</dd>
</dl>
<h2 id="Examples">Examples</h2>
<h3 id="Blob_constructor_example_usage">Blob constructor example usage</h3>
<p>The {{domxref("Blob.Blob", "Blob() constructor")}} allows one to create blobs from other objects. For example, to construct a blob from string:</p>
<pre class="brush: js">var debug = {hello: "world"};
var blob = new Blob([JSON.stringify(debug, null, 2)], {type : 'application/json'});</pre>
<div class="warning">
<p>Before the Blob constructor was available, this could be accomplished through the {{domxref("BlobBuilder")}} API, which is now deprecated:</p>
<pre class="brush: js">var builder = new BlobBuilder();
var fileParts = ['<a id="a"><b id="b">hey!</b></a>'];
builder.append(fileParts[0]);
var myBlob = builder.getBlob('text/xml');
</pre>
</div>
<h3 id="Example_for_creating_a_URL_to_a_typed_array_using_a_blob">Example for creating a URL to a typed array using a blob</h3>
<p>The following code:</p>
<pre class="brush: js">var typedArray = GetTheTypedArraySomehow();
var blob = new Blob([typedArray.buffer], {type: 'application/octet-stream'}); // 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.
</pre>
<h3 id="Example_for_extracting_data_from_a_Blob">Example for extracting data from a Blob</h3>
<p>One way to read content from a Blob is to use a {{domxref("FileReader")}}. The following code reads the content of a Blob as a typed array:</p>
<pre class="brush: js">var reader = new FileReader();
reader.addEventListener("loadend", function() {
// reader.result contains the contents of blob as a typed array
});
reader.readAsArrayBuffer(blob);</pre>
<p>Another way to read content from a Blob is to use a Response. The following code reads the content of a Blob as text:</p>
<pre class="brush: js">var text = await (new Response(blob)).text();
</pre>
<p>By using other methods of {{domxref("FileReader")}}, it is possible to read the contents of a Blob as a string or a data URL.</p>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('File API','#blob','Blob')}}</td>
<td>{{Spec2('File API')}}</td>
<td>Initial definition</td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<p>{{Compat("api.Blob")}}</p>
<h2 id="See_also">See also</h2>
<ul>
<li>{{domxref("BlobBuilder")}}</li>
<li>{{domxref("FileReader")}}</li>
<li>{{domxref("File")}}</li>
<li>{{domxref("URL.createObjectURL")}}</li>
<li><a href="/en-US/docs/Components.utils.importGlobalProperties">Components.utils.importGlobalProperties</a></li>
</ul>
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