Unable to call asp.net webservice from javascript function

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VB

Javascript

DevExpress

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I have created an "usercontrol" which will accept the zip code and display state and city(returning in json format).
So I have written keyup function for zipcode textbox in javacript.
In javascript side, I want to call the webservice.
Here is my code written in the ascx page:

function OnKeyUP(s, e) {
        var len = txtZip.GetText();
            var newstring = len.replace(/[_-]/g, '');
            if (newstring.length == 5) {              

                $.ajax(
                    {
                        type: "POST",
                        url: '~/CityStateZipCodes.asmx/GetCityStateZipCodes',
                        data: "{'zip':'77853'}",
                        async: false,
                        contentType: "application/json; charset=utf-8",
                        dataType: "json",
                        success: function (msg) {
                            alert(msg);                           
                        },
                        error: function () {
                            alert('error');
                        }
                    });
                }
             }

But I am unable to call the webservice, can someone please help me out how to call, or give an example.