Event.cancelable

The cancelable read-only property of the Event interface indicates whether the event can be canceled, and therefore prevented as if the event never happened. If the event is not cancelable, then its cancelable property will be false and the event listener cannot stop the event from occurring.

Event listeners that handle multiple kinds of events may want to check cancelable before invoking their preventDefault() methods.

Most browser-native events that can be canceled are the ones that result from the user interacting with the page. Canceling the click, scroll, or beforeunload events would prevent the user from clicking on something, scrolling the page, or navigating away from the page, respectively.

Custom events created by other JavaScript code control if they can be canceled when they are created.

bool = event.cancelable;

The result is a Boolean, which is true if the event can be canceled.

For example, browser vendors are proposing that the wheel event can only be canceled the first time the listener is called — any following wheel events cannot be canceled.

function preventScrollWheel(event) {
  if (typeof event.cancelable !== 'boolean' || event.cancelable) {
    // The event can be canceled, so we do so.
    event.preventDefault();
  } else {
    // The event cannot be canceled, so it is not safe
    // to call preventDefault() on it.
    console.warn(`The following event couldn't be canceled:`);
    console.dir(event);
  }
}

document.addEventListener('wheel', preventScrollWheel);

Whether an event can be canceled or not is something that's determined when that event is initialized.

To cancel an event, call the preventDefault() method on the event. This keeps the implementation from executing the default action that is associated with the event.