Для наглядности:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>ГГ</title>
</head>
<body>
<input type="text">
<script>
document.body.onload = function(){
function cal(){alert('gg')}
document.querySelector('input').oninput = cal;
}
document.body.onload = function(){
function cal(){alert('gggg')}
document.querySelector('input').oninput = cal;
}
</script>
</body>
</html>