Warn on positional selectors

[dmethvin]

See discussion in jquery/sizzle#293 . They aren't being removed in jQuery 3.0 but we should warn since they are being deprecated.

[dmethvin]

Can we move this back from the 3.0.0 milestone? Potentially it could still be removed in core 4.0 but I'm concerned that there will be a lot of noise from Migrate in real-world code, especially plugins.

[mgol]

It should be fine. We may very well deprecate more stuff in jQuery 3.1.0
etc. and Migrate updates will be needed to handle those deprecations. If
there's no time for this now, I'm good with postponing.

Michał Gołębiowski

[dmethvin]

For reference, the positional selectors are here along with the recommendation of alternatives. I think this full discussion will end up being big enough that it belongs in a wiki page rather than the warnings document, since there are subtle differences with the alternatives due to counting (0-base for jQuery positional selectors) and the fact that -child selectors don't apply to the entire jQuery set.

jQuery selector	Alternative
:even	.even() method or :nth-child(odd)
:odd	.odd() method or :nth-child(even)
:first	.first() method or :first-child
:last	.last() method or :last-child
:eq()	.eq() method
:gt(n)	.slice(n+1)
:lt(n)	.slice(0, n)

[stof]

These are now announced as deprecated in jQuery 3.4.0: https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/
So warning about them would be great.

[stof]

@dmethvin do .gt() and .lt actually exist ? api.jquery.com does not seem to list them, AFAICT.

[dmethvin]

@stof Oh true, you'd need to use .slice(). I've updated the comment. There used to be an .lt() and .gt() but it was deprecated long ago when .slice() arrived and removed in 1.2.

[timmywil]

:first-child, :last-child, and :nth-child while they work when all of the elements have the same parent are technically not functionally equivalent (edit: I see you already noted this). The methods are the right choice.

Also, we don't have .even or .odd. I don't know if there's a clear replacement for those. Maybe we should add them in 4.0?

[dmethvin]

Wow, I could have sworn we had those, I think we did long ago. It's trivial to add .even and .odd, either ourselves or by showing the code in the migration guide, but in many cases :nth-child or :nth-of-type do the trick in a selector as long as you're aware of the 0 vs. 1 offset.