Uncover the Mystery: What is a Blob in JavaScript?

what is a blob javascript

As a web developer, you may have come across the term “Blob” while working with JavaScript. But what exactly is a Blob? Put simply, a Blob is a data type in JavaScript that represents a file-like object of immutable, raw data.

A Blob can be composed of any type of data, including text, images, videos, and audio files. It is commonly used for handling large amounts of data, such as when uploading or downloading files from a server.

If you’re new to JavaScript or unfamiliar with Blobs, don’t worry. In this article, we’ll explain everything you need to know about Blobs in JavaScript, including their purpose, how they are used, and the data type they represent.

Key Takeaways

  • Blobs are a data type in JavaScript that represent a file-like object of immutable, raw data.
  • A Blob can be composed of any type of data, including text, images, videos, and audio files.
  • Blobs are commonly used for handling large amounts of data, such as when uploading or downloading files from a server.

Understanding the Blob Object in JavaScript

Now that we know what a Blob is, let’s dive deeper into the Blob object in JavaScript. The Blob object represents a file-like object of immutable, raw data. It is created by passing an array of MIME types and an array of data to the Blob constructor.

The Blob object has several properties, including size, type, and arrayBuffer(). The size property returns the size of the data in bytes, while the type property returns the MIME type of the data. The arrayBuffer() method is useful for reading the data as an ArrayBuffer.

To create a Blob object, you can use the following code:

var blob = new Blob(arrayOfData, {‘type’: mimeType});

The arrayOfData is an array of data, which can be an ArrayBuffer, a Blob, or a string. The mimeType is the type of data contained in the Blob.

You can also manipulate Blobs using various methods, such as slice(), which returns a new Blob object containing a portion of the data from the original Blob. The following code shows how to use the slice() method:

var newBlob = blob.slice(start, end, contentType);

The start parameter is the starting index of the Blob, while the end parameter is the ending index. The contentType parameter is the content type of the new Blob.

Overall, learning how to create and manipulate Blobs in JavaScript can be incredibly useful for web development. By understanding the Blob object and its methods, you can work with raw data in a more efficient and effective way.

Working with Blobs in JavaScript

Blobs are a versatile and powerful data type in JavaScript, allowing you to store and retrieve large amounts of binary data. Here, we’ll explore some of the ways you can work with blobs in your JavaScript code.

How to use Blob in JavaScript

To use blobs in JavaScript, you first need to create a new Blob object. This can be done using the Blob() constructor function, which takes an array of data as its argument.

Note: The Blob constructor function is available in most modern browsers, including Chrome, Firefox, Safari, and Edge.

Here’s an example of how to create a new blob with some text data:

Code Result
var data = ["Hello, world!"];
var blob = new Blob(data);
The blob variable now contains a new blob object with the text “Hello, world!” inside.

You can also create blobs from binary data, such as images or audio files, using the same Blob constructor function. Just pass in an array of binary data instead of text data.

Working with Blobs in JavaScript

Once you’ve created a blob object, there are many different ways you can work with it in your code. Here are just a few examples:

  • Reading blob data: You can read the data stored inside a blob using the FileReader object, which allows you to read files and blobs asynchronously.
  • Writing blob data: You can write data to a blob using the BlobBuilder or the newer Blob() constructor function. This is useful for creating new blobs or modifying existing ones.
  • Displaying images stored in blobs: You can display images stored in blobs by creating a new Image() object and setting its src property to a blob URL. This allows you to display images without needing to first upload them to a server.
  • Handling blob downloads and uploads: You can download or upload blobs using JavaScript by creating a new XMLHttpRequest() object and sending a request to a server. This is useful for handling file uploads or downloads without needing to navigate away from the current page.


Working with blobs in JavaScript can be a powerful way to store and manipulate binary data in your code. With the right tools and techniques, you can create and manipulate blobs to build dynamic and engaging web applications.

An Example of Blob Usage in JavaScript

Let’s say we want to allow users to upload images to our website. The traditional approach is to store the images on the server and load them from there, but that can slow down page loading times and increase server bandwidth usage. Instead, we can use blobs to store the images locally and display them in our web page.

First, we need to create an input element where users can select the image they want to upload:

<input type="file" id="imageInput">

Next, we’ll add an event listener that triggers when the user selects an image:

const imageInput = document.getElementById('imageInput');

imageInput.addEventListener('change', () => {
  const file = imageInput.files[0];
  const blob = new Blob([file], { type: file.type });

  const url = URL.createObjectURL(blob);

  const img = document.createElement('img');
  img.src = url;


Here’s what’s happening in the code:

  1. We select the file the user uploaded and create a new Blob object from it.
  2. We create a URL for the blob using the URL.createObjectURL() method.
  3. We create a new image element and set its source to the blob URL.
  4. We append the image to the document body.

Now, when the user selects an image, we create a new blob from it, generate a URL for the blob, and display the image on the page without having to upload it to the server. This can significantly improve page loading times and reduce server bandwidth usage.

As you can see, blobs are a powerful tool for working with binary data in JavaScript, and can be used to streamline web development. Start experimenting with blobs and see what you can create!


In conclusion, blobs in JavaScript are essential for handling and storing complex data types in web applications. They allow for efficient transfer of data between the client and server, making it possible to store images, videos, and other media types directly in the database.

We have learned that the Blob object in JavaScript provides a range of methods and properties that allow developers to create, manipulate, and work with blobs in various ways. By using techniques like reading and writing blob data, displaying images stored in blobs, and handling blob downloads and uploads, developers can unlock the full potential of blobs in their code.

Next Steps

If you are interested in learning more about blobs in JavaScript, there are many resources available online to help you deepen your understanding and skills. Some useful keywords to search for include “javascript blob tutorial,” “blob object in javascript,” and “blob data type in javascript.”

By incorporating blobs into your web development projects, you can streamline data transfer and storage, improving the performance and functionality of your applications. We hope this article has helped you uncover the mystery of blobs in JavaScript and inspired you to explore their potential further.


Q: What is a Blob in JavaScript?

A: A Blob is a data type in JavaScript that represents raw binary data. It can store large amounts of data, such as images, videos, or audio files, and is commonly used for file manipulation and data handling in web development.

Q: How do I create a Blob object in JavaScript?

A: To create a Blob object in JavaScript, you can use the Blob constructor and pass in an array of data or a BlobPart. You can also specify the MIME type of the data if needed. Once created, the Blob object can be used for various operations like reading, writing, or displaying the data.

Q: How can I read data from a Blob in JavaScript?

A: Reading data from a Blob in JavaScript involves using the FileReader object. You can instantiate a FileReader, call its readAsDataURL or readAsText method, and pass in the Blob object you want to read from. The result will be a data URL or text representation of the Blob’s contents, which you can then use in your code.

Q: How can I display an image stored in a Blob in JavaScript?

A: To display an image stored in a Blob in JavaScript, you can create an HTML element and set its src attribute to a data URL generated from the Blob using the FileReader object. This will allow you to show the image directly in the browser without the need for a server request.

Q: Can I download a Blob in JavaScript?

A: Yes, you can download a Blob in JavaScript by creating a URL object using the URL.createObjectURL method and setting it as the href attribute of an anchor () element. When the user clicks on the anchor, the Blob will be downloaded to their device.

Q: How can I upload a Blob in JavaScript?

A: Uploading a Blob in JavaScript typically involves sending it as part of a form submission or using the Fetch API to make a POST request. You can retrieve the Blob data using the FileReader object and include it as a form field or as the request body, depending on your server-side requirements.

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