I feel like I'm always confused by one thing when it comes to handling DOM events in JavaScript.
When should I call stopPropagation() on the event object? When should I call preventDefault() on the event object? Should I return false?
Suppose I want to handle a click event on an element.
By default an event on a DOM element is fired on the specific element clicked (say, a button) and will be propagated to all its parent elements tree, unless it’s stopped.
I want to make sure the event is handled in my event handler, and it will "stop" there.
You can stop the propagation by calling stopPropagation() method of an Event object, usually at the end of the event handler:
const link = document.getElementById('my-link')
link.addEventListener('mousedown', event => {
// process the event
// ...
event.stopPropagation()
})
Calling the preventDefault() method of the event object will cancel the default handling that the browser is programmed to execute.
Opening a new page on an a element click event, for example.
Or submitting a form on the submit event.
Calling preventDefault() is what you need to do to completely customize the action. Perhaps by creating a fetch request to load some JSON instead of opening a new page on a link click.
Other event handlers defined on this same element will execute. Unless you call event.stopImmediatePropagation().
This is especially confusing to former (or current) jQuery developers. In jQuery, returning false from an event handler automatically called Event.preventDefault and Event.stopPropagation for us.
In vanilla JavaScript, return false in an event handler does nothing.