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Editorial Team
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Asked: 2026-05-16T01:58:21+00:00

Given Two text Boxes. (TextBox1 & TextBox2), I want add two numbers (using the

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Given Two text Boxes. (TextBox1 & TextBox2), I want add two numbers (using the two text boxes) and show the result in the thrid textbox (TextBox3) instantly i.e without pressing a asp.net button. This is to be done using Javascript. I’m new to javascript so dont have much idea.

This is the asp.net page.

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>

        Box1
        <br />
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <br />
        <br />
        Box2<br />
        <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
        <br />
        <br />
        Box3<br />
        <asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>

    </div>
    </form>
</body>
</html>
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1 Answer

  1. Editorial Team

    W- try this (it’s all based around the js ‘onkeyup’ event handler):

    [edit 2] – changed event handler yet again to onkeyup. you’d need to do a little check in the sum(0 function that the textbox value was a number (isNan).

    <html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Untitled Page</title>
</head>
<body>
    First Number :<input id="txtFirstNumber" type="text" onkeyup="sum();" /><br />
    Second Number:<input id="txtSecondNumber" type="text" onkeyup="sum();" /><br />
    Third Number:<input id="txtThirdNumber" type="text" /><br />
    <input id="changeWatcher" type="text" />
</body>

<script type="text/javascript">

    function sum() {
        var txtFirstNumberValue = document.getElementById('txtFirstNumber').value;
        var txtSecondNumberValue = document.getElementById('txtSecondNumber').value;
        var result = parseInt(txtFirstNumberValue) + parseInt(txtSecondNumberValue);
        if(!isNaN(result))
        {
            document.getElementById('txtThirdNumber').value = result;
            //alert(parseInt(txtFirstNumberValue) + parseInt(txtSecondNumberValue));
        document.getElementById('changeWatcher').value = new Date();
        }
    }

</script>

</html>
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