Solana RPC WebSocket subscriptions

import WebSocket from "ws";

const apiKey = process.env.SOLANA_RPC_API_KEY;
const account = process.env.ACCOUNT_ADDRESS;
if (!apiKey || !account) {
  throw new Error("Set SOLANA_RPC_API_KEY and ACCOUNT_ADDRESS");
}
const endpoint = new URL("wss://rpc-data.solanatracker.io/");
endpoint.searchParams.set("api_key", apiKey);
const socket = new WebSocket(endpoint);
let subscriptionId;
let stopping = false;
let closeTimer;

function unsubscribe() {
  socket.send(JSON.stringify({
    jsonrpc: "2.0", id: 2,
    method: "accountUnsubscribe", params: [subscriptionId],
  }));
}

socket.on("open", () => {
  socket.send(JSON.stringify({
    jsonrpc: "2.0", id: 1, method: "accountSubscribe",
    params: [account, { encoding: "base64", commitment: "confirmed" }],
  }));
});

socket.on("message", raw => {
  let message;
  try { message = JSON.parse(raw.toString()); }
  catch { console.error("Invalid JSON received"); return; }
  if (message.error) {
    console.error("RPC error", message.error);
    socket.close();
    return;
  }
  if (message.id === 1) {
    subscriptionId = message.result;
    console.log("Subscribed", subscriptionId);
    if (stopping) unsubscribe();
  } else if (message.id === 2) {
    console.log("Unsubscribed", message.result);
    socket.close();
  } else if (message.method === "accountNotification" &&
             message.params.subscription === subscriptionId) {
    const { context, value } = message.params.result;
    console.log({ slot: context.slot, lamports: value.lamports });
  }
});

socket.on("error", error => console.error("WebSocket error", error.message));
socket.on("close", () => {
  clearTimeout(closeTimer);
  console.log("Connection closed");
});
process.once("SIGINT", () => {
  stopping = true;
  closeTimer = setTimeout(() => socket.terminate(), 3_000);
  if (socket.readyState === WebSocket.OPEN && subscriptionId !== undefined) {
    unsubscribe();
  } else if (socket.readyState !== WebSocket.OPEN) {
    socket.terminate();
  }
});