Typescript: An index signature cannot be a union of strings

[peaBerberian]

TypeScript Version: 2.7.0

Code

type FruitName = "apple"|"pear";
interface IFruitInventory {
  [key : FruitName] : number;
}

Expected behavior:

interface IFruitInventory {
  [key : FruitName] : number;
}

Should be equivalent to

interface IFruitInventory {
  apple? : number;
  pear? : number;
}

Actual behavior:
Error : An index signature parameter type must be 'string' or 'number' when defining the _IFruitInventory_ interface.

It may be a duplicate, but I could not find another one with the same problem (most of the others I saw were about enums).

[pelotom]

The syntax you’re looking for is

type IFruitInventory = {
  [K in FruitName]: number
}

Or, if you want them to all be optional fields as you’ve indicated,

type IFruitInventory = {
  [K in FruitName]?: number
}

The first can also be expressed as Record<FruitName, number>, and the second by Partial<Record<FruitName, number>>.

[peaBerberian]

My bad, thanks a lot.