Use XMLHttpRequest to Call ASP.NET Web API

Most of the times developers use jQuery $.ajax() to call ASP.NET Web API from the client side script. At times, however, you may need to use plain JavaScript to invoke the Web API. Consider a situation wherein you wish to call Web API from HTML5 Web Worker. Now, you can't use jQuery to accomplish your task because DOM access is not allowed inside a web worker. Another reason might be that your project don't have any dependency on jQuery or such libraries. Luckily, calling a Web API using XMLHttpRequest object and plain JavaScript is not hard. This article discusses how that can be done with a sample Customer Web API.

In this example you will use a Customer Web API that does CRUD operations on the Customers table of Northwind database. This Web API is shown below:

public class CustomerController : ApiController
{
    NorthwindEntities db = new NorthwindEntities();

    public List<Customer> Get()
    {
        return db.Customers.ToList();
    }

    public Customer Get(string id)
    {
        return db.Customers.Find(id);
    }

    public string Post(Customer obj)
    {
        db.Customers.Add(obj);
        db.SaveChanges();
        return "Customer added successfully!";
    }

    public string Put(string id,Customer obj)
    {
        db.Entry(obj).State = EntityState.Modified;
        db.SaveChanges();
        return "Customer modified successfully!";
    }

    public string Delete(string id)
    {
        db.Entry(db.Customers.Find(id)).State = 
EntityState.Deleted;
        db.SaveChanges();
        return "Customer deleted successfully!";
    }
}

As you can see CustomerController consists of five methods namely Get(), Get(id), Post(), Put() and Delete(). These methods deal with the GET, GET, POST, PUT and DELETE verbs respectively.

To call this Web API from client side script you will create a JavaScript object - AjaxHelper - that consists of five methods namely SelectAll(), SelectByID(), Insert(), Update() and Delete(). You will then call t these methods to invoke the respective Web API action.

The AjaxHelper object is shown below:

function AjaxHelper(baseUrl)
{
    this._baseUrl = baseUrl;

    var callWebAPI = function (url, verb, data, callback) {

        var xhr = new XMLHttpRequest();

        xhr.onload = function (evt) {
            var data = JSON.parse(evt.target.responseText);
            callback(data);
        }

        xhr.onerror = function () {
            alert("Error while calling Web API");
        }

        xhr.open(verb, url);
        xhr.setRequestHeader("Content-Type",
 "application/json");
        if (data==null) {
            xhr.send();
        }
        else {
            xhr.send(JSON.stringify(data));
        }
    }

    this.SelectAll = function (callback) {
        callWebAPI(this._baseUrl, "GET", null, callback);
    }

    this.SelectByID = function (id, callback) {
        callWebAPI(this._baseUrl + "/" + id, "GET", 
null, callback);
    }

    this.Insert = function (obj, callback) {
        callWebAPI(this._baseUrl, "POST", obj, callback);
    }

    this.Update = function (id, obj, callback) {
        callWebAPI(this._baseUrl + "/" + id, "PUT", obj, callback);
    }

    this.Delete = function (id, callback) {
        callWebAPI(this._baseUrl + "/" + id, "DELETE",
 null, callback);
    }
}

The AjaxHelper object receives base URL of the Web API as its constructor parameter and stores it in a _baseUrl private variable. Then the code shows a private helper function - callWebAPI() - that creates and configures a new instance of XMLHttpRequest object. The callWebAPI() function accepts four parameters - url, verb, data and callback - and configures five aspects of the XMLHttpRequest object:

• It wires a success function by handling the load event of the XMLHttpRequest.
• It wires an error function by handling the error event of the XMLHttpRequest.
• It opens a URL for a specific HTTP verb using open() method.
• It sets the content-type header to application/json since we will be using JSON format for the communication.
• It calls the send() method by passing the data parameter.

Notice the onload event handler. It retrieves the data returned by the Web API using responseText property (which will be in JSON format) and parses it into JavaScript object. The callback parameter supplies a callback function that will be invoked with the Web API call succeeded.

Then the code shows a series of functions of the AjaxHelper object. The SelectAll(), SelectByID(), Insert(), Update() and Delete() methods call the callWebAPI() function by passing the required URL, HTTP verb, data and callback.

You can now use the AjaxHelper object as shown below:

var ajaxHelper = new AjaxHelper("/api/customer");

var selectAllCallback = function (customers) {
    alert(customers.length);
}

var selectByIDCallback = function (customer) {
    alert(customer.CompanyName);
}

var actionCallback = function (msg) {
    alert(msg);
}

//GET
ajaxHelper.SelectAll(selectAllCallback);
ajaxHelper.SelectByID("ALFKI", selectByIDCallback);

//POST
var obj = {
    CustomerID: "ABCDE",
    CompanyName: "Company 1",
    ContactName: "Contact 1",
    Country: "USA"
}
ajaxHelper.Insert(obj, actionCallback);

//PUT
obj.CompanyName = "Company 2";
ajaxHelper.Update("ABCDE", obj, actionCallback);

//DELETE
ajaxHelper.Delete("ABCDE", actionCallback);

As you can see calling a Web API is now a matter of invoking the methods of AjaxHelper object. The callback functions simply display an alert() that confirms that the underlying operation was successful.