Typescript object: How do I restrict the keys to specific strings?

Question

I want to create an object of type Partial, where the keys will be some combination of 'a', 'b', or 'c'. It will not have all 3 keys (edit: but it will at least have one). How do I enforce this in Typescript? Here's more details:

// I have this:
type Keys = 'a' | 'b' | 'c'

// What i want to compile:
let partial: Partial = {'a': true}
let anotherPartial: Partial = {'b': true, 'c': false}

// This requires every key:
type Partial = {
  [key in Keys]: boolean;
}

// This throws Typescript errors, says keys must be strings:
interface Partial = {
  [key: Keys]: boolean;
}

The two methods I've tried above (using mapped types and interfaces) don't achieve what I want. Can anyone help here?

user184994 · Accepted Answer · 2017-11-12T17:21:18.250

54

You can use the ? to make the keys optional, so

interface Partial {
   a?: boolean;
   b?: boolean;
   c?: boolean;
}

Or, you can do this:

type Keys = "a" | "b" | "c";

type Test = {
    [K in Keys]?: boolean
}

edited Nov 12 '17 at 17:21

answered Nov 12 '17 at 17:13

user184994

17,791
1
46
52

4

Thanks! Is there a way to programmatically do this using the `Keys` type? And also, what if I need to require that at least a single key exists? – the_lrner Nov 12 '17 at 17:15
9

How to restrict that _at least_ one key exists? – maninak Mar 27 '19 at 13:46
1

@the_lrner https://stackoverflow.com/a/59213781 should help – Aidin Nov 09 '20 at 13:55

score 1 · Answer 2 · answered Jan 31 '23 at 23:24

1

Another way to do this is...

// I have these keys
type Keys = 'a' | 'b' | 'c'

// This type requires every key:
type WithAllKeys = {
  [key in Keys]: boolean;
}

// This will have a Partial key
interface WithPartialKeys: Partial<WithAllKeys>;

answered Jan 31 '23 at 23:24

Martin

161
1
6

Typescript object: How do I restrict the keys to specific strings?

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