send Uint16Array form browser to node.js via binary sockets

Question

I'm trying to send typed arrays (Uint16Array) from a browser to node.js trough a binary socket but I'm getting incoherent values when I receive them in node. I read lot of documentation but there is something that I don't understand...

Here is the browser-side code :

var ws = new WebSocket('ws://127.0.0.1:8080');
ws.binaryType = "arraybuffer";

var message = new Uint16Array(3);
message[0] = 123;
message[1] = 456;
message[2] = 789;

ws.onopen = function() {
    ws.send(message.buffer);
};

And this is the server code (I'm using https://github.com/einaros/ws) :

    var WebSocketServer = require('ws').Server;
    var wss = new WebSocketServer({port: 8080});
    wss.on('connection', function(ws) {

        ws.on('message', function(buffer, flags) {

            var message = new Uint16Array(buffer);
            console.log(message[0]); // 123
            console.log(message[1]); // 0
            console.log(message[2]); // 200

        });
    });

Would you have any idea where this variation could come from? Thanks to all !

Dimitri

Paul · Accepted Answer · 2019-07-12 17:33:57Z

Hilariously late answer, but it looks like ws flattens typed arrays into byte-sized buffers. 123 comes through fine because it fits into the low byte of the uint16. 0 is the high byte of 123. 200 is just the low byte of 456:

456 & 0xFF // 200

The buffer you receive in your server's message handler should be twice as long as the Uint16Array you sent. You'll have to manually reconstruct the uint16's yourself.

var WebSocketServer = require('ws').Server;
var wss = new WebSocketServer({port: 8080});

// Pre-allocate so we're not incurring allocation overhead on every message
var message = new Uint16Array(3);

wss.on('connection', function(ws) {
    ws.on('message', function(buffer, flags) {
        for (var i = 0; i < uint16Array.length; i++) {
            message[i] = buffer[2*i] | message[2*i+1] << 8;
        }

        console.log(message[0]); // 123
        console.log(message[1]); // 456
        console.log(message[2]); // 789
    });
});

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