Question

有理由对其中哪些内容会更快?

$( ul.dropdown a ).first().click(function(event) {
    event.stopPropagation();
    return false;
});

或

$( .dropdown > li > a ).click(function(event) {           
    event.preventDefault();
});

是否有任何区别?

Answer 1

www.un.org/spanish/ecosoc 它们有两个不同的东西:。

I guess you re asking about > V.S. space:
> is fatser then space, as it goes one level deep only.

The tip with selectors is make the right side of it more precise than the left.
Read more here

Answer 2

$( .dropdown ).find( a ).on( click , function(e) {           
    e.preventDefault();
});