code: function(loader) {
    // public
    this.maxRecursion = 5;
    this.loader = (typeof loader === 'function') ? loader : null;
  
    // internal
    this._schemaRegistry = new SchemaRegistry();
    this._customFormatHandlers = {};
  
    // _refsRequested is an object where the key is the normalized ID
    // of the schema ref that was requested, and the value is a
    // promise for the content of that schema.
    this._refsRequested = {};
  }
docs: ****************************************************************** Constructor ******************************************************************