How Can I Efficiently Decode Base64 Data into a Blob in JavaScript?

Decoding Base64 Data into a Blob: A Comprehensive Guide for JavaScript Developers

In JavaScript, working with base64-encoded binary data is a common task. To display this data to the user, it's often necessary to create a Blob object. This article provides a thorough guide on how to decode base64 data into a Blob, addressing the specific challenge of handling prohibitively large data URLs.

Converting Base64 to a Byte Array

To decode a base64 string into binary data, we can leverage the atob function. It returns a new string with one character for each byte in the encoded data.

To convert these characters to their corresponding byte values, we iterate over each character using the charCodeAt method, resulting in an array of byte numbers.

const byteCharacters = atob(b64Data);
const byteNumbers = new Array(byteCharacters.length);
for (let i = 0; i < byteCharacters.length; i++) {
  byteNumbers[i] = byteCharacters.charCodeAt(i);
}

Creating a Typed Byte Array

We can use the Uint8Array constructor to convert the byte numbers array into a typed byte array, which is an array-like object representing the underlying binary data.

const byteArray = new Uint8Array(byteNumbers);

Converting to a Blob

Finally, we can wrap the typed byte array in an array and pass it to the Blob constructor to create a Blob object.

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

Optimizing Performance

While the above code works, its performance can be improved by processing the byte characters in smaller slices rather than all at once.

The b64toBlob function below demonstrates this improvement, with a slice size of 512 bytes, which has been found to provide a good balance between memory usage and performance.

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;
};

Full Example

To demonstrate the usage of b64toBlob, we can create an image element and set its source to a Blob URL created from a base64-encoded image.

const blob = b64toBlob(b64Data, 'image/png');
const blobUrl = URL.createObjectURL(blob);

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

By following these steps and utilizing the provided code snippets, you can effectively decode base64 data into Blob objects in JavaScript, enabling you to display large binary data to users without encountering the limitations of data URLs.

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