Using a string as an index in typescript

Question

I am trying to type the following

    interface IStudentType {
      [key: `${Students}`]: IStudent | IStudentMaths| IStudentPhysics
     }

The issue I get is TS1268: An index signature parameter type must be 'string', 'number', 'symbol', or a template literal type.

Fair enough so I try

type StudentCategories = 'Mature' | 'Graduate' | 'Fresher' // these are the keys in the data

interface IStudentType {
  [key: `${StudentCategories}`]: IStudent | IStudentMaths| IStudentPhysics
}

TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.

Any ideas how to fix this?

Answer 1

If you have a union of keys, you should use Record

type StudentCategories = 'Mature' | 'Graduate' | 'Fresher' // these are the keys in the data

type IStudent = {
    tag: 'IStudent '
}

type IStudentMaths = {
    tag: 'IStudentMaths'
}
type IStudentPhysics = {
    tag: 'IStudentPhysics'
}

type StudentType = Record<StudentCategories, IStudent | IStudentMaths | IStudentPhysics>

Playground

It is possible to use Symbol and Template String Pattern Index Signatures and PR but then you need to add a prefix:

interface StudentType {
    [prop: `${string}-${StudentCategories}`]: IStudent | IStudentMaths | IStudentPhysics
}