ASP.NET Web API で JSON を取得する方法

<input type="button" id="apiButton" class="btn" value="Web Api Test" />

<script>
    function asyncCall() {
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function () {
            if ((xhr.readyState === 4) && (xhr.status === 200)) {
                var json = xhr.response;
                console.log(json);
            }
        }
        xhr.open("GET", "/api/values", true);
        xhr.send(null);
    }

    var button1 = document.getElementById("apiButton");
    button1.addEventListener("click", asyncCall, false);
</script>

public class Item
{
    public int Id { get; set; }

    public string Name { get; set; }

    public DateTime RegisteredOn { get; set; }

    public int CategoryId { get; set; }
}

public class ValuesController : ApiController
{
    // GET api/values
    public IEnumerable<Item> Get()
    {
        List<Item> items = new List<Item>();
        items.Add(new Item { Id = 1, Name = "Toshihiko", CategoryId = 1, RegisteredOn = DateTime.Today });
        items.Add(new Item { Id = 2, Name = "Takashi", CategoryId = 1, RegisteredOn = DateTime.Today });
        return items;
    }
}