[lhorie]

The difference between something like Rust->ASM and TS->JS is that the former actually emits runtime machine code to deal with e.g. an Option, whereas in TS, the compiler is happy to emit the exact same runtime code for both `const o: Foo = JSON.parse('null')` and `const o: Foo | null = JSON.parse('null')` without throwing a compile error.

Rust makes it exceptionally "hard" to write unsafe code, by making it blindingly obvious when you're doing it, whereas in TS it can be very challenging to spot unsoundness, especially considering that the audience of the language is not type system scholars.

[status_quo69]

I won't argue that getting strong code in TS is easy but I definitely don't think it's as hard as you're making it out to be. That being said, it's more of a limitation of the underlying execution context in my opinion, and you have to realize that it's not just jS that's out to get you but the whole ecosystem. But that's the expectation in all languages, even in Rust. In production optimizations all bets are off and you're on your own if your app dies.

For example of what I mean by the platform being out to get you, the second parameter to JSON.parse is a recovery function. So it'll happily parse anything and return anything. Again, should be "unknown" but it was typed before unknown was a thing.

    JSON.parse('{ "fooDate": 123456789 }', (key, value) => !key.includes('Date') ? value : new Date(value));

    JSON.parse('123456789', (key, value) => new Date(value))

I actually just learned about this today, since I was doing some digging. Horrifying stuff for a typed language to get around. FWIW, we've made it an explicit lint error to use the `any` type, and enabled --noImplicitAny and --noImplicitReturns. That's been pretty solid for our codebase and because I/O is limited to a few edges, we can call out those edges for type guards.