How to have type check for strings that we know will contains only certain values.
Example: const binary = '1010000101000'; We know that binary values represented in decimal would only be 1’s & 0’s. To have a better type check, what would be a good type definition for these kinds of values.
type Binary = '0' | '1'; wouldn’t work, because these would be representing only single characters of the string. But the idea is how to have an interface/type for the whole string that we know would contain only certain types of characters in a string.
The question is not about choosing interface for binary values, it’s how to declare/define types for predefined string values.
>Solution :
You can use a recursive typing :
type BinDigit = "0" | "1"
type OnlyBinDigit<S> =
S extends ""
? unknown
: S extends `${BinDigit}${infer Tail}`
? OnlyBinDigit<Tail>
: never
function onlyBinDigit<S extends string>(s: S & OnlyBinDigit<S>) {
}
onlyBinDigit("01010101010011"); // OK
onlyBinDigit("010101012"); // NOK