prompt()

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
</head>
<body>
<script>
var val0, val1;
window.onload=function() {
val0=parseFloat(prompt('enter a value', ''));
val1=parseFloat(prompt('enter a value', ''));
alert('Sum = '+(val0+val1));
}
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
</head>
<body>
<script>
var val0, val1;
window.onload=function() {
val0=parseFloat(prompt('enter a value', ''));
val1=parseFloat(prompt('enter a value', ''));
alert('Sum = '+(val0+val1));
}
</script>
</body>
</html>