Disabling and Enabling Update Btn

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HTML

<link rel="stylesheet" href="http://cdn.kendostatic.com/2014.1.528/styles/kendo.common.min.css">
<link rel="stylesheet" href="http://cdn.kendostatic.com/2014.1.528/styles/kendo.default.min.css">
<script src="http://cdn.kendostatic.com/2014.1.528/js/kendo.all.min.js"></script>
            <div id="grid"></div>

JavaScript

$(document).ready(function () {
    var crudServiceBaseUrl = "http://demos.telerik.com/kendo-ui/service",
        dataSource = new kendo.data.DataSource({
            transport: {
                read: {
                    url: crudServiceBaseUrl + "/Products",
                    dataType: "jsonp"
                },
                update: {
                    url: crudServiceBaseUrl + "/Products/Update",
                    dataType: "jsonp"
                },
                destroy: {
                    url: crudServiceBaseUrl + "/Products/Destroy",
                    dataType: "jsonp"
                },
                create: {
                    url: crudServiceBaseUrl + "/Products/Create",
                    dataType: "jsonp"
                },
                parameterMap: function (options, operation) {
                    if (operation !== "read" && options.models) {
                        return {
                            models: kendo.stringify(options.models)
                        };
                    }
                }
            },
            batch: true,
            pageSize: 20,
            schema: {
                model: {
                    id: "ProductID",
                    fields: {
                        ProductID: {
                            editable: false,
                            nullable: true
                        },
                        ProductName: {
                            validation: {
                                required: true
                            }
                        },
                        UnitPrice: {
                            type: "number",
                            validation: {
                                required: true,
                                min: 1
                            }
                        },
                        Discontinued: {
                            type: "boolean"
                        },
                        UnitsInStock:...