In their documentation here : http://getbootstrap.com/docs/4.0/components/forms/#checkboxes-and-radios-1
It's never mentioned how to properly use the custom checkboxes, using the inspector it doesn't seem like any class is added, and the value is always "on".
It's also not mentioned how do I change their state via html or javascript.
JsFiddle : https://jsfiddle.net/2n40w13k/1/
Js :
$('.custom-control-input').on('change', evt => {
console.log(evt.target.value);
})