watch-party/frontend/lib/reconnecting-web-socket.mjs

export default class ReconnectingWebSocket {
  constructor(url){
    if(url instanceof URL) {
      this.url = url;
    } else {
      this.url = new URL(url)
    }
    this.connected = false;
    this._eventTarget = new EventTarget();
    this._backoff = 250; // milliseconds, doubled before use
	this._lastConnect = 0;
    this._socket = null;
    this._unsent = [];
    this._connect(true);
  }
  _connect(first) {
    if(this._socket) try { this._socket.close() } catch (e) {};
    try {
      this._socket = new WebSocket(this.url.href);
    } catch (e) {
      this._reconnecting = false;
      return this._reconnect()
    }
    this._socket.addEventListener("close", () => this._reconnect())
    this._socket.addEventListener("error", () => this._reconnect())
    this._socket.addEventListener("message", ({data}) => this._eventTarget.dispatchEvent(new MessageEvent("message", {data})))
    this._socket.addEventListener("open", e => {
      if(first) this._eventTarget.dispatchEvent(new Event("open"));
      if(this._reconnecting) this._eventTarget.dispatchEvent(new Event("reconnected"));
      this._reconnecting = false;
      this._backoff = 250;
      this.connected = true;
      while(this._unsent.length > 0) this._socket.send(this._unsent.shift())
    })
  }
  _reconnect(){
    if(this._reconnecting) return;
    this._eventTarget.dispatchEvent(new Event("reconnecting"));
    this._reconnecting = true;
    this.connected = false;
    this._backoff *= 2; // exponential backoff
    setTimeout(() => {
      this._connect();
    }, Math.floor(this._backoff+(Math.random()*this._backoff*0.25)-(this._backoff*0.125)))
  }
  send(message) {
    if(this.connected) {
      this._socket.send(message);
    } else {
      this._unsent.push(message);
    }
  }
  addEventListener(...a) {
    return this._eventTarget.addEventListener(...a)
  }
  removeEventListener(...a) {
    return this._eventTarget.removeEventListener(...a)
  }
}