Get started with 33% off your first certification using code: 33OFFNEW

Working with Blobs in JavaScript

3 min read
Published on 12th February 2024

Blobs, or Binary Large Objects, in JavaScript are immutable objects that represent raw data. They can be used for a variety of purposes, such as file operations, image processing, and more. One practical example we've discussed lately is using blobs to chunk uploads in JavaScript.

Let’s dive into the world of Blobs and understand how to work with them effectively in JavaScript.

What is a Blob?

A Blob in JavaScript is an object that represents data that isn't necessarily in a JavaScript-native format. The File interface is based on Blob, inheriting Blob's functionality and expanding it to support user system file operations.

Creating a Blob

You can create a Blob by using the Blob constructor, which takes an array of data and options as parameters. The data can be any JavaScript objects, such as BufferSource, Blob, String, etc.


const data = new Uint8Array([72, 101, 108, 108, 111]); // Hello in ASCII
const blob = new Blob([data], { type: 'text/plain' });

Reading a Blob

To read data from a Blob, you can use the FileReader API. This API reads the contents of Blobs and Files and returns data in various formats.

Example: Reading Blob as Text:

const reader = new FileReader();
reader.onload = function(e) {
    console.log(; // Outputs: Hello

Creating a Blob URL

Blob URLs can be created using the URL.createObjectURL() method. This URL can be used as a reference to the Blob object in your application, such as setting it as the source of an <img> tag.


const imageUrl = URL.createObjectURL(blob);
document.getElementById('myImage').src = imageUrl;

Working with Files

Since Files are an extension of Blobs, all operations that you can do with Blobs are also applicable to File objects. This is particularly useful for file operations, like reading user-uploaded files.

Example: Handling File Uploads:

const inputElement = document.querySelector('input[type="file"]');
inputElement.addEventListener('change', handleFiles, false);

function handleFiles() {
    const fileList = this.files; // fileList is a FileList of File objects
    const file = fileList[0];

    // Read the file content
    const reader = new FileReader();
    reader.onload = function(e) {

Converting Between Blobs and Arrays

Sometimes, you may need to convert a Blob to an ArrayBuffer for processing binary data or vice versa.

Example: Blob to ArrayBuffer Conversion:

const blobToArrayBuffer = async (blob) => {
    return new Promise((resolve, reject) => {
        const reader = new FileReader();
        reader.onload = () => resolve(reader.result);
        reader.onerror = () => reject(reader.error);

Use Cases for Blobs

  • File Operations: Handling user-uploaded files, such as images, videos, or documents.
  • Image Processing: Creating image thumbnails, converting image formats, etc.
  • Data Streaming: Streaming large data chunks, like video or audio data.
  • Data Storage: Storing binary data locally using IndexedDB or other mechanisms.

Best Practices

  • Memory Management: Revoke Blob URLs when not needed using URL.revokeObjectURL() to free up memory.
  • Error Handling: Always handle errors, especially when reading from or converting Blobs.
  • Performance Considerations: Be mindful of performance when dealing with large Blobs, as operations can be memory-intensive.

Blobs in a nuteshell

Blobs are a versatile and powerful feature in JavaScript for handling binary data. Understanding how to create, read, and manipulate Blobs opens up a range of possibilities for web developers in data processing and management.