Method subscribe

lib/src/store.ts:320–333

(subject: string, callback: callback)

   */
  // TODO: consider subscribing to properties, maybe add a second subscribe function, use that in useValue
  subscribe(subject: string, callback: callback): void {
    if (subject == undefined) {
      throw Error('Cannot subscribe to undefined subject');
      return;
    }
    let callbackArray = this.subscribers.get(subject);
    if (callbackArray == undefined) {
      // Only subscribe once
      this.subscribeWebSocket(subject);
      callbackArray = [];
    }
    callbackArray.push(callback);
    this.subscribers.set(subject, callbackArray);
  }

  subscribeWebSocket(subject: string): void {
    if (subject == unknownSubject) {