How to write an interface with base value and unknown amount of dynamic properties?

I have an object structure which could be as follows:

const myObj = {
  callback : ( val : boolean ) => concole.log( val ),
  string1  : 'foo',
  string2  : 'bar',
  string3  : 'bla',
  // eventually string4, string5 etc.
}

Is there a way to define an interface where the object has an optional property callback and optional properties string + num?

Like:

interface MyInterface {
  callback                                ?: ( val ?: boolean ) => void;
  [ ( must be "string" + num ) : string ] ?: string
} 

So the only allowed props would be callback followed by string1, string2 and so forth wit no limit like string200.

Is this possible?

>Solution :

We can now use template literal types as keys in index signatures:

interface MyInterface {
    callback?: (val?: boolean) => void;
    [key: `string${bigint}`]: string;
}

I’ve used bigint instead of number so only integers are allowed.

This allows keys like string1 and string123 but not string or string1.2 (or the cases @jcalz showed 😛).

Playground