[рони]

Ну во первых вы получаете на выходе хрень.

var data = '[{"id":"14"},{"rows":"8","min_left":"1","max_right":"16"},{"id":"6","NSRight":"12","NSLeft":"2"},{"id":"12","NSRight":"6","NSLeft":"4"},{"id":"14","NSRight":"8","NSLeft":"8"},{"id":"12"},{"id":"14"},{"id":"6","rep":"48","max_right":"16"},{"id":"11","rep":"48","max_right":"16"},{"id":"12","rep":"48","max_right":"16"},{"id":"15","rep":"48","max_right":"16"}]';
function foo(json, key) {
    var value;
    JSON.parse(json).some(function(obj, i) {
        return obj[key] != void 0 ? (value = obj[key], true) : false
    });
    return value
};
alert(foo(data,'rows') );