Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script>
function checkField(val)
{
alert("The input value has changed. The new value is: " + val);
}
</script>
</head>
<body>

<p>Modify the text in the input field, then click outside the field to fire onchange.</p>
Enter some text: <input type="text" name="txt" value="Hello" onchange="checkField(this.value)">

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.