Why firing a defined event with dispatchEvent doesn't obey the bubbling behavior of events?

问题

I'm confused with the script below:

var event = new Event('shazam');

document.body.addEventListener('shazam',function(){
    alert('body');
});

document.addEventListener('shazam',function(){
    alert('document');
});

window.addEventListener('shazam',function(){
    alert('window');
});

document.body.dispatchEvent(event);

When I run this script on my browser, I just get the alert('body'); event. but if i set the capturing parameter of addEventListener (the third optional parameter) to true, all the alerts captured in order they should.

Why the shazam event doesn't bubble up ?

回答1:

You need to set the bubbles property to true, and you have to do this during the construction:

var event = new Event('shazam', { bubbles: true });

or the old way with initEvent, passing true as the second argument to allow bubble:

event.initEvent('shazam', true);

MDN Doc

来源：https://stackoverflow.com/questions/23048322/why-firing-a-defined-event-with-dispatchevent-doesnt-obey-the-bubbling-behavior