function check() {
if (document.getElementById("myCheck").checked == true) {
document.getElementById("myCheck").checked = true;
}
}<!DOCTYPE html>
<html>
<body>
Checkbox: <input type="checkbox" id="myCheck" onclick="check()">
</body>
</html>You can remove the condition if (document.getElementById("myCheck").checked == true) if you want to keep it checked once it has been checked for the first time.
function check() {
document.getElementById("myCheck").checked = true;
}<!DOCTYPE html>
<html>
<body>
Checkbox: <input type="checkbox" id="myCheck" onclick="check()">
</body>
</html>Disable the checkbox after it has been clicked once.
document.getElementById("myCheck").disabled = true;
with inputs we use onchange
const checkbox = document.getElementById('my-checkbox')
checkbox.checked = true
checkbox.onchange = function() {
checkbox.checked = true
}