> It's also not a type. How do you distinguish epoch from number?
I dunno, how do you distinguish "April" (the month) vs "April" (someone's first name) or 18 (kg) vs 18 ($) vs 18 (-th of the month) vs 18 (page number)?
> It is lower fidelity (seconds vs milliseconds).
This isn't a problem if you have a conforming implementation (2^52 milliseconds is over 100'000 years), but as nitrogen pointed out, you do apparently have to worry about that.
What the original poster means is that if I go JSON.parse(...) then how does that know to give me an object with a Date type instead of a Number? Answer: It can't.
This is a frequent gotcha with Typescript, where even if your type is declared with a Date field, Javascript won't care when it deserializes it, as that type information is all erased and not available at runtime.
(Also, Number in JS being floating point and all, it lacks the integer precision for high resolution timestamps - if you start serialising 64 bit timestamps and expect a JS-style runtime to do good things with them it doesn't end well.)
> how does that know to give me an object with a Date type instead of a Number
Ah, thanks, that makes sense; I'd forgotten that Javascript had a built-in Date type. (Strictly speaking, then, you ought to be able to write something like `{"time":new Date(...)}`, but that obviously doesn't work in practice.)
Since Typescript is mentioned, might as well mention the awesome io-ts[1] library. It gives you both runtime validation and static type safety with very simple syntax.
> I dunno, how do you distinguish "April" (the month) vs "April" (someone's first name) or 18 (kg) vs 18 ($) vs 18 (-th of the month) vs 18 (page number)?
Types! Extensible types. Like I said above, Transit is a good solution to this. With a fixed set of types, arbitrary data will always have ambiguity.
Floats are ugly because the distance between any two successive numbers are not the same. This makes them inappropriate for discretized time and anything accounting-related (money).
Epoch floats and doubles are not good for timestamps, as the further we get from 1970 the less precise they become.
Current precision with a 32-bit float (JSON/JavaScript are 64-bit usually, though in non-JS it's common to use Bigdecimal in slight non-compliance) is much worse than one second.
