The nodeValue property is used to get the node value. You need to specify the node.
Example
You can try to run the following code to learn how to get nodeValue property.
<!DOCTYPE html>
<html>
<body>
<p>Get the node value</p>
<button>Demo Button Text</button>
<script>
var val = document.getElementsByTagName("BUTTON")[0];
var res = val.childNodes[0].nodeValue;
document.write("<br>Node Value: "+res);
</script>
</body>
</html>