WIP: perf(jqLite): use classList for add/removeClass

[rodyhaddad]

Continuation of #7759
Using classList.apply caused problems in IE 10/11.

I believe we still want these changes for performance, but not yet.
So keeping them here for being considered later.

[caitp]

@mzgol would know this better than me, but I'm fairly certain I recall jQuery opting to avoid classList because it hurt performance rather than helped it, in addition to requiring multiple code paths.

I'm not sure this is actually a performance "fix", possibly a regression, although it's not likely that we can actually measure a significant difference given that class operations are not super common

[caitp]

and yeah, (old) opera (~v12-ish) and possibly old IE browsers don't deal with the multiple args to classList apis correctly :(

[rodyhaddad]

This is here because we won't merge them from #7759 for the next release.

We might consider not merging them at all later on.

[lgalfaso]

Is this the same as #7759 9db0cdf ?

[lgalfaso]

@rodyhaddad you beat me while I was writing my comment :)

[rodyhaddad]

@lgalfaso Yes, didn't want to lose track of it :-)

[caitp]

anyways, I am not big on having piles of paths for this stuff, if it's going to be done, we should decide which path to take before binding jqLite (and this kind of sucks for maintenance, too)

[IgorMinar]

we are going to skip this one until IE has better support

[mgol]

Sorry to be late for the party but yeah, apart from IE9 not supporting classList which even jQuery 2 still supports performance is not better. The confusion might come from the fact that classList performance in some browsers (like Chrome) was better for some time but that was when only a single className argument was supported. Once support for multiple arguments was added, performance went back to jQuery levels (I suppose nothing really more performant can happen natively than what jQuery currently does).

classList is definitely a much nicer interface for people that need to type it frequently by hand but for libraries, where you write once and use a lot it doesn't create any significant advantage (and it introduces some drawbacks like not being able to control spacing/trimming of the className property value etc.). And it's certainly not worth introducing two code paths.

IMO it'll make sense to switch to classList once all supported browsers support it fully (i.e. by accepting multiple params, not one like IE right now) but not before.

[rodyhaddad]

@mzgol Thanks for all the info!