Fix adding undefined to class

[mmarkelov]

Fix #502

[taion]

this should be outside the conditional, no? we want to e.g. reset old values

[mmarkelov]

@taion I think that it is unnecessary to add falsy value to object and run addClass function, i can remove this conditional if you think that it will be better approach

[taion]

well, we want to clear any previous value that was there, no? e.g. if this already ran a transition, and the class mappings are dynamic, then we don't want to keep around an old stale value from the last time the transition ran?

[mmarkelov]

className will be undefined for this case (when doneClassName is undefined), so if it is undefined we could pass through:

this.appliedClasses[type][phase] = className

appliedClasses is used in removeClasses function, and it also check for undefined

also we can pass through:

addClass(node, className)

cause it check if className is falsy.
So I suppose it is equivalent, but with this condition, we can skip addClass and pass undefined value to appliedClasses

[taion]

i guess we wipe it in

react-transition-group/src/CSSTransition.js

Line 193 in 5e2c1c4

this.appliedClasses[type] = {};

anyway

[taion]

do we need this change? this seems like essentially a separate thing. does it hurt to leave an empty class attribute around? we wouldn't know if e.g. the node originally had a class="", would we?

[mmarkelov]

@taion youp. I don't know if it is right approach. I add it to cleanup for this specific case, cause it just add class to div, like so:

<div class></div>
<div class></div>
<div class></div>

Maybe there is another way to fix this?
It doesn't hurt to add class attribute, so I can delete this part

[taion]

I think it's harmless.

[gpbl]

Pinging @jquense – would be nice to get this merged :) Thanks!

[jquense]

🎉 This PR is included in version 4.4.0 🎉

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