What Is A Blob Javascript

Javascript Programming

Have you ever heard of a blob in JavaScript? It may sound like a strange term, but it is actually quite useful when it comes to handling binary data in the browser. In this article, I’ll dive deep into what a blob is and how it can be used in JavaScript.

What is a Blob?

A Blob, short for Binary Large Object, is a data type in JavaScript that represents a collection of binary data. It can be anything from an image or audio file to a video or a PDF document. Blobs are particularly useful when you need to work with binary data in a browser environment, such as uploading or downloading files.

Think of a Blob as a container that holds the binary data. It has properties and methods that allow you to manipulate and interact with the data it contains.

Creating a Blob

There are several ways to create a Blob in JavaScript. One way is to use the Blob() constructor function, which takes an array of ArrayBuffer, ArrayBufferView, or DOMString objects as its argument.

Here’s an example:

const data = new Blob([arrayBuffer, uint8Array, 'Hello, world!']);

In this example, we pass an array containing an ArrayBuffer, a Uint8Array, and a string to the Blob() constructor. The resulting blob, stored in the data variable, contains the binary data represented by these objects.

Working with Blobs

Once you have a Blob, you can perform various operations on it. Here are some of the most commonly used methods:

  • size: Returns the size of the blob in bytes.
  • type: Returns the MIME type of the blob.
  • slice(): Creates a new Blob that contains a subset of the original blob’s data.

For example, if you have a Blob representing an image file and you want to create a thumbnail version of it, you can use the slice() method to extract a portion of the image data and create a new Blob with the thumbnail image.

Using Blobs for File Operations

Blobs are commonly used in file-related operations, such as uploading and downloading files. Here’s an example:

// Uploading a file
const fileInput = document.getElementById('file-input');
const file = fileInput.files[0];
const fileBlob = new Blob([file]);

// Downloading a file
const downloadLink = document.getElementById('download-link');
downloadLink.href = URL.createObjectURL(fileBlob);
downloadLink.download = file.name;

In this example, we first create a Blob from a file selected by the user in an input element. We then use the URL.createObjectURL() method to generate a temporary URL for the Blob, which can be used to download the file. Finally, we set the href and download attributes of a link element to make it downloadable.


Blobs are a powerful tool in JavaScript for handling binary data in the browser. Whether you need to manipulate images, upload files, or work with other types of binary data, understanding how to create and use Blobs can greatly enhance your web development skills. So the next time you encounter a blob in JavaScript, don’t be intimidated! Embrace its capabilities and unlock new possibilities in your web applications.