JSFiddle - React, Tailwind, and code Playground

JavaScript

if (!checkIfCustomerIsValid(event, AddCustomer)) {
        event.preventDefault();
        return false;
}

function checkIfCustomerIsValid(event, callbackOnValid) {

  // code that was already there, the name and address check

  var _mobNo;
    if ($('#txtMobile').val() == '') return false;

    var _unq = $.ajax({
        url: '../Autocomplete.asmx/IsMobileUnique',
        type: 'GET',
        contentType: 'application/json; charset=utf8',
        dataType: 'JSON',
        data: "mobileNo='" + $('#txtMobile').val() + "'",
        async: false,
        timeout: 2000,
        success: function (res) { if (res.d) return false; else return true; },
        error: function (res) { alert('some error occurred when checking mobile no'); }
    }), chained = _unq.then(function (data) { if (data.d == false) { alert('mobile no already exists!'); $('#txtMobile').focus(); return false; } return callbackOnValid(); });

}