Array Iterator in Javascript

arrayIterator.js

/*************************************************************
 * Adds iterator to javascript Arrays
 *************************************************************/

/**
 * Returns previous element in the iteration
 * @return {[object]} [Returns null if iteration has not started or if limit has reached]
 */
Array.prototype.prev = function(){
	if(isNaN(this.index) || this.index <=0){
		return null;
	}
	return this[--this.index];
}

/**
 * Returns next element in the iteration
 * @return {[object]} [Returns null if iteration limit been has reached]
 */
Array.prototype.next = function(){
	if(isNaN(this.index)){
		this.index = 0;
	}
	if(this.index >= this.length){
		return null;
	}
	return this[this.index++];
}

/**
 * Returns whether or not the iteration has more elements
 * @return {[boolean]} [Returns true if the iteration has more elements]
 */
Array.prototype.hasNext = function(){
	if(this.index < this.length){
		return true;
	}
	return false;
}

doesn't iteration over key in any array instance now print hasNext, next, and prev as three always-present keys? Seems dangerous.
minimal example: https://gist.github.com/endel/4136334