FTX Websockets Javascript

ftx_websockets.js

const symbols = [
    "BTC-PERP",
    "BTC/USD",
    "ETH-PERP",
    "ETH/USD",
];

const ws_init = function () {
    ws = new WebSocket("wss://ftx.com/ws/");

    ws.onopen = function () {
        for (s of symbols) {
            ws.send(`{"op": "subscribe", "channel": "trades", "market": "${s}"}`);
        }
    };

    ws.onclose = function () {
        setTimeout(ws_init, 5000);
    };

    ws.onerror = function (error) {
        console.log('WS error: ' + error);
    };

    ws.onmessage = function (e) {
        let data = JSON.parse(e.data);
        if (data.channel == "trades" && data.type == "update") {
            handleTrades(data);
        }
    };

    const ping = function () {
        ws.send('{"op":"ping"}');
    };
    setInterval(ping, 10000);
}
ws_init();

const handleTrades = function (data) {
    if (data.market == "BTC/USD") {
        btc_usd = data.data[0].price;  // set the btc global for calculations, used in app.js
    }
    $("#btcPrice").text(`${data.market}: ${data.data[0].price}`);
}

You forgot const WebSocket = require('ws');