JavaScript:The Class of an Object

function is(type, obj) { 
    var clas = Object.prototype.toString.call(obj).slice(8, -1); 
    return obj !== undefined && obj !== null && clas === type; 
} 
 
is('String', 'test'); // true 
is('String', new String('test')); // true 

 

 

   In the above example, Object.prototype.toString gets called with the value of this being set to the object whose [[Class]] value should be retrieved.