Enable/Disable onbuttonclick
HTML
<body>
First name: <input type="text" name="fname"><br>
Last name: <input id="lname" type="text" name="lname" disabled><br>
<button id="button">Submit</button>
</body>JavaScript
document.getElementById('button').onclick = enable;
function enable(){
document.getElementById('lname').disabled = !document.getElementById('lname').disabled;
}