I would suggest that today, you're best off using TypeScript for anything more complex in terms of applications development. Will probably play around more with Deno bundling if I get the time, though currently using Parcel, which is nice enough, but slow.
I didn't like TS at first, but using the most recent version has been relatively pleasant and working with less experienced devs has been almost required in order to do further refactors. I killed a month trying to do a complex refactor ad-hoc, in circling around, I was able to convert the entire project to TS in a couple days, introduce typing for core state and a few other areas which allowed me to do the rest of the refactor in a couple days. That sold me on TS, as long as I don't HAVE to type everything, or jump through the hoops as I did on my first experience with TS, which was combined with Angular projects.
The casting behavior is definitely a foot-gun, but it's powerful as I mention for validation and ETL type workflows. The "falsy" values list is literally my guide marker in interviews for JS devs. It's usually a pretty good benchmark for how well a developer understands the language itself..
Totally agreed on the Date object. Absolutely horrible, and moment, while useful was huge and most other hacks aren't all that great either. The Date object really should be extended as probably the next major bump in usability in the system. Even if it was just extended enough for parity for what C#/.Net offers for their DateTime/DateTimeOffset then other libraries could flush it out and be much smaller.
One useful thing to know, is that assigning an object property to undefined will skip that property as part of json serialization and is faster than delete on the property I'll often do something like return JSON.stringify(Object.assign({}, original, {propToHide: undefined}) in node APIs. This is about the only useful bit from undefined I can think of off hand.
Global variable assignment by default is definitely a flaw.