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Creating a BLOB from a Base64 string in JavaScript

In this blog, we will learn how to handle a common scenario encountered by software engineers: converting a Base64 string to a BLOB in JavaScript. This skill proves valuable in various situations, particularly when there’s a need to store images or files in a database. The upcoming sections of this blog post will delve into the process of creating a BLOB from a Base64 string using JavaScript.
By Saturn Cloud | | Miscellaneous | Updated:

As a software engineer, you may come across a situation where you need to convert a Base64 string to a BLOB in JavaScript. This can be useful in many scenarios, such as when you need to store an image or a file in a database. In this blog post, we will discuss how to create a BLOB from a Base64 string in JavaScript.

Table of Contents

  1. What is a Base64 string?
  2. What is a BLOB?
  3. Converting a Base64 string to a BLOB in JavaScript
  4. Best Practices
  5. Common Errors and How to Handle Them
  6. Example
  7. Conclusion

What is a Base64 string?

A Base64 string is a way of representing binary data in ASCII format. It consists of a sequence of characters that represent a sequence of bytes. Each character in the string represents 6 bits of data. The total number of characters in the string is a multiple of 4. Base64 encoding is often used in web applications to transmit images, files, and other binary data.

What is a BLOB?

A BLOB (Binary Large Object) is a type of data that can store binary data such as images, videos, and files. BLOBs are often used in databases to store large amounts of binary data.

Converting a Base64 string to a BLOB in JavaScript

To convert a Base64 string to a BLOB in JavaScript, we can use the atob() and Blob() functions.

The atob() function decodes a Base64 string into a binary data string. It takes a Base64 string as input and returns a string of binary data.

The Blob() function creates a new BLOB object. It takes an array of binary data as input and returns a new BLOB object.

Here’s an example of how to convert a Base64 string to a BLOB in JavaScript:

const byteCharacters = atob(base64String);
const byteArrays = [];

for (let i = 0; i < byteCharacters.length; i++) {
    byteArrays.push(byteCharacters.charCodeAt(i));
}

const byteArray = new Uint8Array(byteArrays);object

This code snippet converts a Base64-encoded string (base64String) into a Binary Large Object (Blob) in JavaScript. Initially, the atob function is used to decode the Base64 string into a series of binary characters (byteCharacters). Subsequently, a loop iterates through each character, obtaining its Unicode value, and populates an array (byteArrays) with these numeric values. The Uint8Array constructor is then employed to create an array of 8-bit unsigned integers (byteArray) from the obtained numeric values. Finally, a Blob is generated using the Blob constructor, taking the Uint8Array as input and allowing the specification of a content type, resulting in a Blob suitable for various applications, such as saving binary data as files.

Best Practices

  • Specify Content Type: When creating a BLOB, always specify the content type using the Blob constructor. This helps browsers interpret the data correctly.

  • Handle Asynchronous Operations: If decoding or processing the Base64 string involves asynchronous operations, ensure proper error handling and use async/await or Promises.

  • Cross-Browser Compatibility: Test your code across different browsers to ensure compatibility. Different browsers may have variations in their implementation of JavaScript functions.

Common Errors and How to Handle Them

Error 1: Invalid Base64 String

One common error is providing an invalid Base64 string. To handle this, check the validity of the string before attempting to decode it.

Error 2: Data URI Mismatch

Ensure that the data URI and the content type specified in the Blob constructor match. Mismatched types can lead to unexpected behavior.

Error 3: Cross-Origin Security Issues

When working with Base64 strings from external sources, be aware of cross-origin security restrictions. Ensure that your application has the necessary permissions to access external resources.

Example

Let’s create a simple html file that has a function to decode base64 of Hello world and save it to a text file.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Base64 to Blob and Save as Text File</title>
</head>
<body>

<script>
// Function to convert Base64 to Blob
function base64ToBlob(base64String, contentType = '') {
    const byteCharacters = atob(base64String);
    const byteArrays = [];

    for (let i = 0; i < byteCharacters.length; i++) {
        byteArrays.push(byteCharacters.charCodeAt(i));
    }

    const byteArray = new Uint8Array(byteArrays);
    return new Blob([byteArray], { type: contentType });
}

// Function to save Blob as a text file
function saveBlobAsTextFile(blob, fileName) {
    const link = document.createElement('a');
    link.href = URL.createObjectURL(blob);
    link.download = fileName;
    link.click();
}

// Example Base64 string of a text file
const base64TextFile = "SGVsbG8gd29ybGQ=";  // "Hello world"

// Specify the content type (text/plain for a text file)
const contentType = "text/plain";

// Convert Base64 to Blob
const blob = base64ToBlob(base64TextFile, contentType);

// Save Blob as a text file
saveBlobAsTextFile(blob, "example.txt");
</script>

</body>
</html>

Open this html file on your browser, it will download a text file that has Hello world inside.

Conclusion

In this blog post, we discussed how to create a BLOB from a Base64 string in JavaScript. We learned that a Base64 string is a way of representing binary data in ASCII format, and a BLOB is a type of data that can store binary data such as images, videos, and files. We also learned that we can use the atob() and Blob() functions to convert a Base64 string to a BLOB in JavaScript.

By following the steps outlined in this post, you can easily convert a Base64 string to a BLOB in JavaScript and use it in your web applications. This can be particularly useful when working with databases that store binary data.

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