Saving the file was as easy as creating a new Buffer with the Uint8Array object :

// chunk is the Uint8Array object
fs.appendFile(path, Buffer.from(chunk), function (err) {
    if (err) {
      fut.throw(err);
    } else {
      fut.return(chunk.length);
    }
});

Have you tried first converting it into a node.js. Buffer? (this is the native node.js Buffer interface, whereas ArrayBuffer is the interface for the browser and not completely supported for node.js write operations).

Something along the line of this should help:

all= fs.createWriteStream("out."+imgtype);

for(i=0; i<end; i++){
    var buffer = new Buffer( new Uint8Array(picarray[i]) );
    all.write(buffer);
}
all.end();

Instances of Buffer are also instances of Uint8Array in node.js 4.x and higher. Thus, the most efficient solution is to access the buffer's own .buffer property directly, as per https://stackoverflow.com/a/31394257/1375574. The Buffer constructor also takes an ArrayBufferView argument if you need to go the other direction.

Note that this will not create a copy, which means that writes to any ArrayBufferView will write through to the original Buffer instance.

From Buffer to ArrayBuffer:

function toArrayBuffer(buffer) {
  const arrayBuffer = new ArrayBuffer(buffer.length);
  const view = new Uint8Array(arrayBuffer);
  for (let i = 0; i < buffer.length; ++i) {
    view[i] = buffer[i];
  }
  return arrayBuffer;
}

From ArrayBuffer to Buffer:

function toBuffer(arrayBuffer) {
  const buffer = Buffer.alloc(arrayBuffer.byteLength);
  const view = new Uint8Array(arrayBuffer);
  for (let i = 0; i < buffer.length; ++i) {
    buffer[i] = view[i];
  }
  return buffer;
}

The answer by Leonenko cites/copies the correct JavaScript documentation, but it turns out that writeFile doesn't play nicely with a Uint8Array -- it simply writes the bytes out as numbers:

"84,104,101,32,102,105,114,115,..."

To get it to work, one has to wrap the Uint8Array in a Buffer:

fs.writeFile('testfile',new Buffer(ui8a),...)