Display image from blob using javascript and websockets

I'm currently working on a WebSocket application that is displaying images send by a C++ server. I've seen a couple of topics around there but I can't seem to get rid of this error in Firefox:

Image corrupt or truncated: data:image/png;base64,[some data]

Here's the Javascript code I'm using to display my blob:

socket.onmessage = function(msg) {
    var blob = msg.data;

    var reader = new FileReader();
    reader.onloadend = function() {
        var string = reader.result;
        var buffer = Base64.encode(string);
        var data = "data:image/png;base64,"+buffer;

        var image = document.getElementById('image');
        image.src = data;

I'm using the image of a red dot that I found on this topic: https://stackoverflow.com/a/4478878/1464608 And the Base64 class is from here: https://stackoverflow.com/a/246813/1464608

But the base64 outcome I get doesn't match and Firefox retrieves me an error of the image being corrupted.

I know this ain't much informations but I don't have a clue where to look :/ Any help is more than welcome!!


I think the cleanest solution would be to change the base64 encoder to operate directly on a Uint8Array instead of a string.

Important: You'll need to set the binaryType of the web socket to "arraybuffer" for this.

The onmessage method should look like this:

socket.onmessage = function(msg) {
    var arrayBuffer = msg.data;
    var bytes = new Uint8Array(arrayBuffer);

    var image = document.getElementById('image');
    image.src = 'data:image/png;base64,'+encode(bytes);

The converted encoder should then look like this (based on https://stackoverflow.com/a/246813/1464608):

// public method for encoding an Uint8Array to base64
function encode (input) {
    var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    var output = "";
    var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
    var i = 0;

    while (i < input.length) {
        chr1 = input[i++];
        chr2 = i < input.length ? input[i++] : Number.NaN; // Not sure if the index 
        chr3 = i < input.length ? input[i++] : Number.NaN; // checks are needed here

        enc1 = chr1 >> 2;
        enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
        enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
        enc4 = chr3 & 63;

        if (isNaN(chr2)) {
            enc3 = enc4 = 64;
        } else if (isNaN(chr3)) {
            enc4 = 64;
        output += keyStr.charAt(enc1) + keyStr.charAt(enc2) +
                  keyStr.charAt(enc3) + keyStr.charAt(enc4);
    return output;

Thanks, it's working great!!

So I figure I'd share my final javascript code:

var socket = new WebSocket('ws://'+host+':'+port, protocol);
socket.binaryType = 'arraybuffer';

try {
    socket.onopen = function() {
        document.getElementById('status').style.backgroundColor = '#40ff40';
        document.getElementById('status').textContent = 'Connection opened';

    socket.onmessage = function(msg) {
        var arrayBuffer = msg.data;
        var bytes = new Uint8Array(arrayBuffer);

        var image = document.getElementById('image');
        image.src = 'data:image/png;base64,'+encode(bytes);

    socket.onclose = function(){
        document.getElementById('status').style.backgroundColor = '#ff4040';
        document.getElementById('status').textContent = 'Connection closed';
} catch(exception) {

don't really understand why the blob version is so tricky but this did the trick!

You may write it much simpler:

socket.onmessage = function(msg) {
   var arrayBuffer = msg.data;
   var bytes = new Uint8Array(arrayBuffer);
   var blob        = new Blob([bytes.buffer]);

   var image = document.getElementById('image');

   var reader = new FileReader();
   reader.onload = function(e) {
       image.src = e.target.result;

Another alternative

let urlObject;

socket.onmessage = function(msg) {
    const arrayBuffer = msg.data;
    const image = document.getElementById('image');

    if (urlObject) {
        URL.revokeObjectURL(urlObject) // only required if you do that multiple times
    urlObject = URL.createObjectURL(new Blob([arrayBuffer]));

    image.src = urlObject;


Thanks to the other answers, I managed to receive a jpeg image by websocket and display it in a new window :

socket.binaryType = "arraybuffer";                 
socket.onmessage = function (msg) 
               {    var bytes = new Uint8Array(msg.data);
                    var blob = new Blob([bytes.buffer]);

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