JavaScript versions ?
in: Settings | Languages & Frameworks | JavaScript
There is button "…" after JavaScript language version drop-down list to add and select JavaScript versions - what it is used for ?

I can not find relevant documentation for this. ("There is no help for this dialog")
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this dialog allows assigning JavaScript language version to specific folders in your project. Please see https://blog.jetbrains.com/webstorm/2017/09/webstorm-2017-3-eap-173-2463/#multiple-js-versions
Thank you.
I didn't search in blog, sorry. Next time I'll search also in blog :)