Some ideas on checking for an isogram in javascript

isogram.js

var Isogram = function(w){

	var word = w.toLowerCase().split('');
	
	this.isIsogram = function(){
		
		var letters = new Object(null);
		
		for(var i = 0; i < word.length; i++){ 
		
			if(letters.hasOwnProperty(word[i])){ return false; }
			else{ letters[word[i]] = i; }
			
		}
		
		return true;
	};
	
	this.isIsogramRecur = function(){
		
		var checkLetterRecur = function(letters){
			
			var head = letters[0];
			var tail = letters.slice(1);
			
			if(tail.length === 0){ return true; }
			else if(tail.indexOf(head) > -1){ return false; }
			else{ return checkLetterRecur(tail); }

			
		};
		
		return checkLetterRecur(word);
	};
};

var word1 = new Isogram('duplicates');
console.log("word1:       " + word1.isIsogram());
console.log("word1 Recur: " + word1.isIsogramRecur());
console.log('');
var word2 = new Isogram('eleven');
console.log("word2:       " + word2.isIsogram());
console.log("word2 Recur: " + word2.isIsogramRecur());