Typescript How to express string concatenation in type system

Question

I would like to make sure, that string contains other string, at transpile time:

type OneOrTwo = "one" | "two"
type StringContainingOneOrTwo = string
let str: StringContainingOneOrTwo = "123" + "one" // ok, contains OneOrTwo
let str2: StringContainingOneOrTwo = "123" + "onesss" // should fail, does not contain OneOrTwo

Answer 1

Typescript How to express string concatenation in type system

You can't do that. Especially with contains e.g. your example

let str2: StringContainingOneOrTwo = "123" + "onesss" // should fail, does not contain OneOrTwo

Actually does contain one i.e. 123 + one + sss. But even with ends with you can't encode that in the typescript type system.