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

'typeof' and 'instanceof' isn't a reliable method to find out the object class, so this is a function that does this in a more trustable way.

Checking Type of Object

JavaScript