Here is the example code to get and use TextBox Values with Javascript:
<html>
<head>
<script type="text/javascript">
function myfunction() {
var first = document.getElementById("textbox1").value;
var second = document.getElementById("textbox2").value;
var answer = parseFloat(first) + parseFloat(second);
var textbox3 = document.getElementById('textbox3');
textbox3.value=answer;
}
</script>
</head>
<body>
<input type="text" name="textbox1" id="textbox1" /> + <input type="text" name="textbox2" id="textbox2" />
<input type="submit" name="button" id="button1" onclick="myfunction()" value="=" />
<br/>
The answer is:<input type="text" name="textbox3" id="textbox3" readonly="true"/>
</body>
</html>