> Is the lack of such a feature in TypeScript (or any language) a deliberate design decision to avoid unnecessary complexity, or due to technical constraints such as performance considerations?It makes a lot of things impossible. For example, if you defined two different types of ranges, OneToFifty and OneToHundred similarly to your PercentType above, the following code would be problematic:
let x: OneToFifty = <...>;
let y: OneToHundred = <...>;
y = x;
Any human programmer would say the third line makes sense because every OneToFifty number is also OneToHundred. But for a compiler, that's impossible to determine because JavaScript code is Turing-complete, and so it can't generally say that one is certainly a subset of the other.
In other words, any two custom-defined types like that would be unassignable from and to each other, making the language much less usable. Now add generics, co-/contravariance, type deduction, etc., and suddenly it becomes clear how much work adding a new type to the type system is; much more than just a boolean function.
That said, TypeScript has a lot of primitives, for example, template string types for five-digit zip codes:
type Digit = '0' | '1' | '2' | '3' | <...> | '9';
type FiveDigitZipCode = `${Digit}${Digit}${Digit}${Digit}${Digit}`;
(Actually, some of these are Turing-complete too, which means type-checking will sometimes fail, but those cases are rare enough for the TS team to deem the tradeoff worth.)
It makes a lot of things impossible. For example, if you defined two different types of ranges, OneToFifty and OneToHundred similarly to your PercentType above, the following code would be problematic:
Any human programmer would say the third line makes sense because every OneToFifty number is also OneToHundred. But for a compiler, that's impossible to determine because JavaScript code is Turing-complete, and so it can't generally say that one is certainly a subset of the other.In other words, any two custom-defined types like that would be unassignable from and to each other, making the language much less usable. Now add generics, co-/contravariance, type deduction, etc., and suddenly it becomes clear how much work adding a new type to the type system is; much more than just a boolean function.
That said, TypeScript has a lot of primitives, for example, template string types for five-digit zip codes:
(Actually, some of these are Turing-complete too, which means type-checking will sometimes fail, but those cases are rare enough for the TS team to deem the tradeoff worth.)