Simple Example:-
<html>
<script type="text/javascript">
function add(){
var x = parseInt( document.getElementById( "input1" ).value );
var y = parseInt( document.getElementById( "input2" ).value );
if(!isNaN(x)&&!isNaN

)
alert(x+y);
}
function sub(){
var x = parseInt( document.getElementById( "input1" ).value );
var y = parseInt( document.getElementById( "input2" ).value );
if(!isNaN(x)&&!isNaN

)
alert(x-y);
}
function mul(){
var x = parseInt( document.getElementById( "input1" ).value );
var y = parseInt( document.getElementById( "input2" ).value );
if(!isNaN(x)&&!isNaN

)
alert(x*y);
}
function div(){
var x = parseInt( document.getElementById( "input1" ).value );
var y = parseInt( document.getElementById( "input2" ).value );
if(!isNaN(x)&&!isNaN

&&y!=0)
alert(x/y);
}
</script>
<body>
<input id="input1" type="text" />
<input id="input2" type="text" />
<br />
<input id="add" type="button" value=" + " onClick="add()" />
<input id="sub" type="button" value=" - " onClick="sub()" />
<input id="mul" type="button" value=" * " onClick="mul()" />
<input id="div" type="button" value=" / " onClick="div()" />
</body>
</html>