Is there a better way to construct types with properties from templated strings?

Question

When I construct this type

type Test<Name extends string> = {
  [key in `a${Name}`]: 88;
  [key in `b${Name}`]: 21;
};

I get the error "A mapped type may not declare properties or methods.ts(7061)" However, the following works fine

type Test2<Name extends string> = {
  [key in `a${Name}`]: 88;
} & {
  [key in `b${Name}`]: 21;
};

Can someone explain the difference?

Also is there a better way to construct these types? It feels odd to use in for only one type.

Answer 1

Why you cannot declare additional properties is covered here: Typescript: Cannot declare additional properties on Mapped types

A short answer: It seems like an unanticipated use case so not implemented.

It's easy just to intersect a mapped and a usual type

type Type = MappedType & UsualType;

You could create a generic type to ease the case:

type AddName<T, Name extends string> = {
    [K in keyof T as K extends string ? `${K}${Name}` : never]: T[K];
}

type AB = AddName<{a: 88, b: 21}, 'Name'>;

/* type AB = {
    aName: 88;
    bName: 21;
}*/

Playground