How to get quantity multiply price in amount textbox by jque

Question

Problem

 

How to get quantity multiply price in amount textbox by jquery asp.net .

 

I work on web application web form asp.net .

 

AND I need to get qunatity * price and result display on Amount

 

when i try to do that i get error

 

JavaScript runtime error: Unable to get property 'children' of undefined or null

 

reference .

 

this error on line var quantity = tableRows[2].children[0].innerHTML;

<script src="scripts/jquery-3.3.1.js"></script>
<script src="scripts/jquery-3.3.1.min.js"></script>
<script type="text/javascript">
$(function () {
$(".CssPrice").on('change keyup paste', function () {
var textBox = this;
var Price = $(textBox).val();
var tableRows = $(textBox).parent().parent().parent().parent().children();
if (Price != "") {
var quantity = tableRows[2].children[0].innerHTML;
var itemTotal = price * quantity;
tableRows[4].children[0].innerHTML = itemTotal;
}
else
tableRows[4].children[0].innerHTML = "";
});
});
</script>
<asp:GridView ID="GridItems" runat="server" AutoGenerateColumns="false" OnRowDataBound="GridItems_RowDataBound" OnRowEditing="OnRowEditing" OnRowCancelingEdit="OnRowCancelingEdit" OnDataBound="GridItems_DataBound" >
<Columns>
<asp:BoundField DataField="RowNumber" HeaderText="Row Number" />
<asp:TemplateField HeaderText="ItemCode">
<ItemTemplate>
<asp:DropDownList ID="ddlItemCode" runat="server" Width="150px" AutoPostBack="true" >
</asp:DropDownList>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Quantity">
<ItemTemplate>
<asp:TextBox ID="TxtQuantity" runat="server" Text='<%# Bind("Quantity") %>' ></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Price">
<ItemTemplate>
<asp:TextBox ID="TxtPrice" runat="server" Text='<%# Bind("Price") %>' OnTextChanged="TxtPrice_TextChanged" AutoPostBack="true" CssClass="CssPrice" ></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Amount" >
<ItemTemplate>
<asp:TextBox ID="TxtAmount" runat="server" Text='<%# Bind("Amount") %>' >
</asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>