code: function Subscriber( pid, sid, wpcom ) {
  	if ( ! sid ) {
  		throw new Error( '`side id` is not correctly defined' );
  	}
  
  	if ( ! pid ) {
  		throw new Error( '`post id` is not correctly defined' );
  	}
  
  	if ( ! ( this instanceof Subscriber ) ) {
  		return new Subscriber( pid, sid, wpcom );
  	}
  
  	this.wpcom = wpcom;
  	this._pid = pid;
  	this._sid = sid;
  }
docs: `Subscriber` constructor.
  
  @param {String} pid - post identifier
  @param {String} sid - site identifier
  @param {WPCOM} wpcom - wpcom instance
  @api public