Typescript define array that can contain only specific strings

Question

I'm confused to why following errors, I have character interface

interface Character {
 race: "ORC" | "ELF" | "HUMAN" | "DWARF"
}

And another interface tavern

interface Tavern {
  races: Character['race'][]
}

Idea here is that races is an array of strings that can only be "ORC" | "ELF" | "HUMAN" | "DWARF"

For some reason I get an error when I use it like this

const tavern: Tavern = {
   races: ["ORC", "ELF", "HUMAN", "DWARF"]
}

Error reads as follows

[ts] Type '{ races: string[] }' is not assignable to type 'Tavern'. Types of property 'races' are incompatible. Type 'string[]' is not assignable to type '("HUMAN" | "ORC" | "ELF" | "DWARF")[]'. Type 'string' is not assignable to type '"HUMAN" | "ORC" | "ELF" | "DWARF"'.

2.9.2, very weird, I'm not sure what else could be causing that error, will look into it more — Ilja
– Ilja, Commented Jul 24, 2018 at 18:23
Yeah, I didn't get any error either with your exact code. I was going to suggest a helpers function to infer the literal types instead of string but something este is at work typescriptlang.org/play/… — Titian Cernicova-Dragomir
– Titian Cernicova-Dragomir, Commented Jul 24, 2018 at 19:21

Daniel Khoroshko · Accepted Answer · 2018-07-24 18:30:11Z

11

this is an old typescript story, you will most likely have to do this:

const tavern: Tavern = {
   races: ["ORC", "ELF", "HUMAN", "DWARF"] as Array<Character['race']>
}

possibly

type Race = "ORC" | "ELF"

const tavern: Tavern = {
   races: ["ORC" as Race, "ELF" as Race]
}

this should work

enum Race = { ORC, ELF }

interface Tavern {
  races: Race[]
}

const tavern: Tavern = {
   races: [Race.ORC, Race.ELF]
}

edited Jul 24, 2018 at 18:30

answered Jul 24, 2018 at 18:27

Daniel Khoroshko

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2 Comments

Ilja Over a year ago

That first one did the trick. Is there any source or github issue you could reference that explores this in more depth?

Daniel Khoroshko Over a year ago

I am not sure about any references, but the problem is that typescripts treats your array as regular string[] initially. Btw, I have updated the answer - enums might be a good choice here, they can hold values as well like ORC = "ORC" if you want to have sane interop with other systems

Daniel Kaplan · Accepted Answer · 2023-04-13 22:47:11Z

0

I think this may have been a typescript bug when you wrote the question. Typescript v5.0.4 (the latest at the time of this answer) compiles your code without issue:

interface Character {
 race: "ORC" | "ELF" | "HUMAN" | "DWARF"
}

interface Tavern {
  races: Character['race'][]
}

const tavern: Tavern = {
   races: ["ORC", "ELF", "HUMAN", "DWARF"]
}

answered Apr 13, 2023 at 22:47

Daniel Kaplan

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