JavaScript Blob

Blob objects in JavaScript are essential for handling raw data directly from the user or other sources. A Blob object represents immutable raw binary data, making it a cornerstone for applications that require handling files, such as image or document processing. In this detailed guide, we delve into practical uses of Blob objects, provide code examples, and showcase advanced techniques to elevate your JavaScript projects.

Creating and Modifying Blob Objects

Basic Blob Creation

To start working with Blob objects, you must first understand how to create them. A Blob can be created using its constructor, which takes an array of parts and options as arguments. Here's a simple example:

const simpleBlob = new Blob(["Hello, world!"], { type: "text/plain" }); simpleBlob.text().then(console.log);

The second parameter of the Blob constructor is an options object that allows you to specify additional metadata about the Blob being created. This parameter is optional and can include properties such as type. The type property is a string that indicates the MIME type of the Blob's data, helping applications understand how to handle the Blob. For example, setting type to "text/plain" defines the Blob as plain text. There are many other MIME types, too. Such as "image/jpeg" for JPEG images, "application/json" for JSON data, or "audio/mp3" for MP3 audio files. See MIME-Types for more information.

Combining Blobs

Combining multiple Blob objects into a single Blob is straightforward. Blob objects are immutable, so you need to create a new Blob that combines two previous Blobs. This technique is useful when you need to aggregate data from multiple sources:

const blobPart1 = new Blob(["This is "], { type: "text/plain" }); const blobPart2 = new Blob(["a test."], { type: "text/plain" }); const combinedBlob = new Blob([blobPart1, blobPart2], { type: "text/plain" }); const reader = new FileReader(); reader.onload = function() { console.log(reader.result); // Outputs: This is a test. }; reader.readAsText(combinedBlob);

Working with Blob URLs

Generating Blob URLs

Blob URLs allow you to reference Blob objects in your applications as if they were regular URLs. This is particularly useful for creating downloadable links or embedding multimedia content:

const data = new Blob(["Example text"], { type: "text/plain" }); const blobUrl = URL.createObjectURL(data); console.log(blobUrl); // Outputs a URL, e.g., blob: // the last part of the URL changes every time you run it.

Revoking Blob URLs

It's important to manage the lifecycle of Blob URLs by revoking them when they're no longer needed, to free up memory:

const data = new Blob(["Example text"], { type: "text/plain" });
const blobUrl = URL.createObjectURL(data);

// After using the blobUrl 
// ...

URL.revokeObjectURL(blobUrl);  // Cleans up the memory used by the Blob URL

Advanced Blob Handling Techniques

Reading Blob Content

Reading content from a Blob can be done using FileReader. Or you may use the text method in the most recent versions of JavaScript. Here’s how to asynchronously read a Blob as text:

const reader = new FileReader(); reader.onload = function() { console.log(reader.result); // Outputs the content of the Blob }; reader.readAsText(new Blob(["Sample data"], { type: "text/plain" })); new Blob(["Another data"], { type: "text/plain" }).text().then(console.log);

Converting Blob to Base64

Converting a Blob to a Base64 string is essential for transmitting binary data in environments that only support text:

const blobToBase64 = blob => { const reader = new FileReader(); return new Promise((resolve, reject) => { reader.onloadend = () => resolve(reader.result); reader.onerror = reject; reader.readAsDataURL(blob); }); }; blobToBase64(new Blob(["hello"], { type: "text/plain" })) .then(console.log); // Outputs: data:text/plain;base64,aGVsbG8=

Practical Applications of Blob Objects

Blob objects are versatile and can be used in various scenarios:

  • File Uploads: Handling file uploads in JavaScript applications, where the Blob can be used to represent the file data.
  • Image Processing: Manipulating images by creating a Blob from canvas data.
  • Large Data Operations: Managing large binary data efficiently without loading it all into memory at once.


Blob objects are a powerful feature in JavaScript, allowing developers to handle raw binary data efficiently. By understanding how to create, modify, and use Blob objects, you can enhance the functionality of your web applications, making them more robust and user-friendly. With the techniques and examples provided, you're well-equipped to implement Blob objects in your next JavaScript project.

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What is the functionality of a Blob object in JavaScript?

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