How to use the pusher-js.warn function in pusher-js

TravisPusher.prototype.init = function (config, apiService) {
  if (!config.key) {
    // Set up a mock Pusher that ignores the expected methods.
    return this.pusher = {
      subscribe() {
        return {
          bind_global() {}
        };
      },
      unsubscribe() {},
      channel() {}
    };
  }

  this.apiService = apiService;
  Pusher.warn = this.warn.bind(this);

  if (config.debug) {
    Pusher.log = function (message) {
      if (window.console && window.console.log) {
        window.console.log(message);
      }
    };
  }

  let pusherConfig = {
    encrypted: config.encrypted,
    disableStats: true,
    wsHost: config.host,

    authorizer: function (channel, options) {
      return {