An IDL event handler is a way to define an event handler for one element via the on javascript properties known as the Interface Definition Language (IDL) (ie defined in the object interface, in the code)
var btn = document.querySelector('button');
btn.onclick = function() {
console.log('Ouch !');
}
<button class="btn btn-info">Poke me</button>
Note that an event handler can also be set via the on HTML attribute
Because this is a property, only one function can be defined. If more than one function should be triggered for instance for the click above, you should add another event listener
See the handler list section.