var reader = new FileReader();
reader.readAsDataURL(blob); 
reader.onloadend = function() {
  var base64data = reader.result;                
  console.log(base64data);
}

Form the docs readAsDataURL encodes to base64

As an awaitable function:

function blobToBase64(blob) {
  return new Promise((resolve, _) => {
    const reader = new FileReader();
    reader.onloadend = () => resolve(reader.result);
    reader.readAsDataURL(blob);
  });
}

Note: The blob's result cannot be directly decoded as Base64 without first removing the Data-URL declaration preceding the Base64-encoded data. To retrieve only the Base64 encoded string, first remove data:/;base64, from the result.

I believe you need to define onloadend before you start reading the data. It is likely completing the read and firing the null function before you've assigned it. Or more precisely at the moment you call readAsDataURL, onloadend is null.

var reader = new FileReader();
reader.onloadend = () => {
  var base64data = reader.result;                
      console.log(base64data);
}

reader.readAsDataURL(blob); 

This has been resolved. The issue was with the backend API merging the PDFs together. Using Apache PDFBox's pdf merger utility, which accepts input streams and the merged pdf document was successfully rendered by the ng2-pdfjs-viewer library. There was no issue with the above JS code.

The atob function will decode a base64-encoded string into a new string with a character for each byte of the binary data.

const byteCharacters = atob(b64Data);

Each character's code point (charCode) will be the value of the byte. We can create an array of byte values by applying this using the .charCodeAt method 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);
}

You can convert this array of byte values into a real typed byte array by passing it to the Uint8Array constructor.

const byteArray = new Uint8Array(byteNumbers);

This in turn can be converted to a Blob by wrapping it in an array and passing it to the Blob constructor.

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

The code above works. However the performance can be improved a little by processing the byteCharacters in smaller slices, rather than all at once. In my rough testing 512 bytes seems to be a good slice size. This gives us the following function.

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

window.location = blobUrl;

Full Example:

Show code snippet

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);

HERE

const b64 = await fetch(url)
      .then((response) => response.buffer())
      .then((buffer) => {
        const b64 = buffer.toString('base64');
        return b64;
      })
      .catch(console.error);