<script>
	var newObj = {};
	var obj = JSON.parse('{"employees":[{"firstName":"John", "lastName":"Doe"},{"firstName":"John",	"lastName":"Smith"},{"firstName":"Peter", "lastName":"Jones"}]}');
	for (var i = 0, len = obj['employees'].length; i < len; i++) {
		if(obj['employees'][i]['firstName'] in newObj) {
			newObj[obj['employees'][i]['firstName']].push(obj['employees'][i]);
		} else {
			newObj[obj['employees'][i]['firstName']] = [obj['employees'][i]];
		}
	}
	console.log(newObj);
</script>