(JavaScript) Hash map implementation

HashMap.js

function Pair(key, value) {
	this.key = key;
	this.value = value;
	this.next = null;
}

function HashMap(size) {
	size = size || 35;

	var storage = new Array(35);

	function getHash(key, size) {
		// Try round robin 
		var hash = 0;
		var chrCode;
		
		if (key.length === 0) {
			return hash;
		}

		for (var i = 0; i < key.length; i++) {
			chrCode   = key.charCodeAt(i);
		    hash  = ((hash << 5) - hash) + chrCode;
		    //hash |= 0; // Convert to 32bit integer
		}

		return hash % size;
	}

	this.put = function (key, value) {
		var index = getHash(key, size);

		if (!storage[index]) {
			storage[index] = new Pair(key, value);
		} else {
			storage[index].next = new Pair(key, value);
		}
	};

	this.get = function (key) {
		var index = getHash(key, size);	

		if (!storage[index]) {
			return null;
		}

		var head = storage[index];

		while (head) {
			if (head.key === key) {
				return head.value;
			}

			head = head.next;
		}	

		return null;
	}

	this.contains = function (key) {
		// How do you know if it's the same key or collision
		return !!this.get(key);
	}
}

var hashMap = new HashMap(3);
hashMap.put('a', 'Jin Lee');
hashMap.put('b', 'Roy');
hashMap.put('c', 'Max');
hashMap.put('d', 'Bob');
hashMap.put('e', 'Prasanna');
hashMap.put('e', 'George');

console.log(hashMap.contains('a'));
console.log(hashMap.get('a'));
console.log(hashMap.contains('b'));
console.log(hashMap.get('b'));
console.log(hashMap.get('e'));