Display Phonecode on Select Country

<label id="lblCountry">Country*:</label>
    <select class="js-example-basic-single form-control" name="state[]" id="country" class="country" onChange="fnCountryCode(this.value);fnCountry()" >

<option>-Select-</option>
......
</select>
<input type="text" class="form-control" id="txtCode" name="txtCode" placeholder="+61">

function fnCountryCode(strCity)
    {
        document.getElementById("txtCode").value = "";
        if (strCity != 'other') {
            var client = new XMLHttpRequest();
            client.open("GET", "https://restcountries.eu/rest/v2/alpha/" + strCity, true);
            client.send();
            client.onreadystatechange = function () {
                if (client.readyState == 4 && this.status == 200) {
                    var arr = JSON.parse(client.responseText);
                    document.getElementById("txtCode").value = "+" + arr.callingCodes;
                    //alert(arr.callingCodes);
                }
            };
        }
    }