docs: function JSONPPolling (opts) {
    Polling.call(this, opts);
  
    // define global callbacks array if not present
    // we do this here (lazily) to avoid unneeded global pollution
    if (!callbacks) {
      // we need to consider multiple engines in the same page
      if (!global.___eio) global.___eio = [];
      callbacks = global.___eio;
    }
  
    // callback identifier
    this.index = callbacks.length;
  
    // add callback to jsonp global
    var self = this;
    callbacks.push(function (msg) {
      self.onData(msg);
    });
  
    // append to query string
    this.query.j = this.index;
  }