Mapped Type with Prefix for each Property

Question

I would like to create a type that only allows properties that start with a given prefix like that:

const example = {
  '#prop1': () => {},
  '#prop2': () => {}
} 

So each Property must be prefixed by a # in that case. I have played around with »Template Literal Types« within »Mapped Types« like so:

interface WithD {
  [s: `#${string}`]: () => void
}

But that yields this error:

An index signature parameter type must be either 'string' or 'number'.

Is there any way to make that happen?

Answer 1

Like @Nadia Chibrikova said, there is no way to build WithD without knowing example value up front.

The only way to handle it - is to know (be able to infer) the type you want to validate.

type Validation<T extends Record<string, any>> = {
  [Prop in keyof T]: Prop extends `#${string}` ? T[Prop] : never
}

type Assert<T, U> = T extends U ? U extends T ? true : false : false


const example1 = {
  '#prop1': () => { },
  '#prop2': () => { },
}

const example2 = {
  ...example1,
  'prop3': () => { }
}

type Example1 = typeof example1;
type Example2 = typeof example2;


type Result1 = Assert<Example1, Validation<Example1>> // true
type Result2 = Assert<Example2, Validation<Example2>> // false


const isValid = <T extends Validation<T>>(arg: T) => { }
isValid(example1) // ok
isValid(example2) // expected error

Playground

Hence, TS should infer your type either from function argument or from type generic parameter