[onetimeaccount1]

Bing Mobile Friendliness Test correctly renders JS pages. Fetch as BingBot does not currently. I logged a support request through Bing Webmaster and got a reply that their engineering team is working on it, so I would expect Bing to crawl JS sites just like Google in the near future.

[greglindahl]

Executing js is expensive, so you should expect bing to limit its willingness to index such sites, same as google.