recursive function issue

HTML

<div id="mydiv">

</div>

JavaScript

function buildString(json, outstring) {
		var andor = json.condition;
    for (var rule in json.rules) {
        if (json.rules[rule].hasOwnProperty("condition")) {
            buildString(json.rules[rule], outstring);
        } else {
            var field = json.rules[rule].id;
            var operator = json.rules[rule].operator;
            var value = json.rules[rule].value == null ? '' : json.rules[rule].value;
            outstring += field + ' ' + operator + ' ' + value;
            if (rule < json.rules.length - 1) {
            	outstring += ' ' + andor + ' ';
            }
        }
    }
    return outstring;
}

var jsonObj = {"condition":"AND","rules":[{"id":"field1","operator":"!= ''","value":null},{"condition":"AND","rules":[{"id":"field2","operator":"== ''","value":null}]},{"id":"field3","operator":"== ''","value":null}]};

$('#mydiv').text(buildString(jsonObj, ""));