Does anybody know when and if SPFx be available for SharePoint on-premises?
If you are using SP 2013 or SP 2010, it wont be available. If you are using SP 2016, it will be available via Feature Packs in this year.
According to this link
The SharePoint Framework works for SharePoint on-premises and SharePoint Online - SPFx overview but MS documentation is bit confusing sometimes so not 100% sure about this.
SPFx Q&A - According to Wictor Wilen's blog post, it will be available in first half of 2017
Its the next development step on SPFx since it's general available since February 23 this year in Office 365.
"Now that we’ve reached general availability, ensuring continued compatibility across versions will be our focus. We’re also working on the next legs of our product roadmap, including on-premises support and new development scenarios"
When is harder to guess, but it wouldn't be a wild guess to say it will be included in feature pack 2 probably released before the summer.
It's available now with Feature Pack 2 for SharePoint Server 2016:
For version compatibility, refer following article: