But at least formal methods tools can be applied directly to the specification, to show properties like "well, if you carve out this subset of TypeScript, you have something you can safely eval()" or "you can never have type errors with this usage of this construct in this way."
> "well, if you carve out this subset of TypeScript, you have something you can safely eval()" or "you can never have type errors with this usage of this construct in this way."
This kind of certainty is very at odds with TypeScript's design philosophy and the way it (necessarily) works. Because it's overlaid on JavaScript, and JavaScript is wildly dynamic (Proxies, prototype modification, etc), almost nothing is known with absolute certainty. To deal with that, TypeScript makes a lot of "optimistic" assumptions, but holds them lightly (i.e. its assumptions may cause type checks to pass when they shouldn't, but it's not going to lean on them for anything that could really blow up if they're wrong)
IMO it strikes a very good balance given the constraints - not criticizing the designers at all - but TypeScript has to be treated differently than other statically typed languages might be, because nearly everything it knows/says is just a "probably"
