underscore中對于對象相等判斷

var aCtor = a.constructor, bCtor = b.constructor;
  if (aCtor !== bCtor && !(_.isFunction(aCtor) && aCtor instanceof aCtor &&
                           _.isFunction(bCtor) && bCtor instanceof bCtor)
                      && ('constructor' in a && 'constructor' in b)) {
    return false;
  }
 

這里為什么要這么判斷兩個對象是否相等？ 代碼來自underscore中的Equal函數
這里的a，b待判斷的對象