Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<form>
<input type="button" id="btn01" value="OK">
</form>

<p>Click the "Disable" button to disable the "OK" button:</p>

<button onclick="disableElement()">Disable</button>

<script>
function disableElement() {
    document.getElementById("btn01").disabled = true;
}
</script>

</body>
</html>