How to Create a Blob from a Base64 String in JavaScript?

Creating a Blob from a Base64 String in JavaScript

The Problem

You have base64-encoded binary data in a string and want to create a Blob URL containing this data and display it to the user. Specifically, the goal is to create a Blob object from the base64 string, as seen in the code snippet below:

const blob = new Blob(????, {type: contentType});
const blobUrl = URL.createObjectURL(blob);

The Solution

To decode a base64 string to a Blob object in JavaScript, follow these steps:

1. Decode the base64 data: Use the atob function to decode the base64 string into a character string:

   const byteCharacters = atob(b64Data);

2. Convert byte characters to byte values: Create an array of byte values by getting the character code point (charCode) for each character in the string:

   const byteNumbers = new Array(byteCharacters.length);
   for (let i = 0; i < byteCharacters.length; i++) {
       byteNumbers[i] = byteCharacters.charCodeAt(i);
   }

3. Create a typed byte array: Convert the byte values into a real typed byte array using the Uint8Array constructor:

   const byteArray = new Uint8Array(byteNumbers);

4. Wrap in an array and create Blob: Create a Blob object by wrapping the byte array in an array and passing it to the Blob constructor:

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

Performance Optimization

The code above is fully functional, but performance can be improved by slicing the byte characters into smaller chunks before processing.

Full Example

Here's a full example, creating an image element from the Blob:

const b64toBlob = (b64Data, contentType='', sliceSize=512) => {
  const byteCharacters = atob(b64Data);
  const byteArrays = [];

  for (let offset = 0; offset < byteCharacters.length; offset += sliceSize) {
    const slice = byteCharacters.slice(offset, offset + sliceSize);

    const byteNumbers = new Array(slice.length);
    for (let i = 0; i < slice.length; i++) {
      byteNumbers[i] = slice.charCodeAt(i);
    }

    const byteArray = new Uint8Array(byteNumbers);
    byteArrays.push(byteArray);
  }

  const blob = new Blob(byteArrays, {type: contentType});
  return blob;
};

const contentType = 'image/png';
const b64Data = 'iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==';

const blob = b64toBlob(b64Data, contentType);
const blobUrl = URL.createObjectURL(blob);

const img = document.createElement('img');
img.src = blobUrl;
document.body.appendChild(img);

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