gistfile1.js

function assertType(obj, type) {
    return obj.constructor.name === type.name
}

Object.prototype.assertType = function(type) {
    return this.constructor.name === type.name
}

//Preparing test, mock objects
function Book(){};
function Desk(){};
var book = new Book();
//Bleh, you can't do 1.prototype() so I made an `one` variable
var one = 1;

/**
 * With function
 */ 
assertType("test", String); //true
assertType(one, String); //false
assertType(one, Number); //true
assertType(book, Book); //true
assertType(book, Desk); //false
 
assertType([], Array); //true
assertType(function(){}, Function); //true


/**
 * With prototype
 */
"test".assertType(String); //true
one.assertType(String); //false
one.assertType(Number); //true
book.assertType(Book); //true
book.assertType(Desk); //false
 
[].assertType(Array); //true
(function(){}).assertType(Function); //true

/**
 * Tweet => code.
 * jsPerf also available in: http://jsperf.com/asset-type
 * 
 */