<!DOCTYPE html>
<html>
<body>
<p>Click the button to get the value of the onclick attribute of the button element.</p>
<button onclick="myFunction()">Try it</button>
<p><strong>Note:</strong> Internet Explorer 8 and earlier does not support the getNamedItem method.</p>
<p id="demo"></p>
<script>
function myFunction() {
var a = document.getElementsByTagName("BUTTON")[0];
var x = a.attributes.getNamedItem('onclick').value;
window.alert(x);
}
</script>
</body>
</html>