Defining Records with narrowed keys

I couldn't find a way in the docs on object types (https://arktype.io/docs/objects) to define a record with narrowed keys. For example keys with no special symbols in them:

BAD

{
  "$userName": "bob"
}

{
  "$userName": "bob"
}

{
  "$userName": "bob"
}

{
  "$userName": "bob"
}

GOOD

{
  "userName": "bob",
}

{
  "userName": "bob",
}

{
  "userName": "bob",
}

{
  "userName": "bob",
}

Seems like there should be a way, but I haven't been able to locate it.

ArkType

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Ccowboyd I couldn't find a way in the docs on object types (https://arktype.io/docs/objec...

TizzySaurus•6/12/25, 1:46 PM

You can use index signatures for anything regex-compatible

TizzySaurus•6/12/25, 1:47 PM

type({'/[a-z]+/': 'unknown'}); // allow any key comprised entirely of lowercase letters, with any value

type({'/[a-z]+/': 'unknown'}); // allow any key comprised entirely of lowercase letters, with any value

TTizzySaurus ```ts type({'/[a-z]+/': 'unknown'}); // allow any key comprised entirely of lowe...

cowboydOP•6/12/25, 1:48 PM

nice! Is error messaging configurable on that in the case of a failure?

Ccowboyd nice! Is error messaging configurable on that in the case of a failure?

TizzySaurus•6/12/25, 1:49 PM

Iirc you can use

.configure

.configure

for that

TizzySaurus•6/12/25, 1:51 PM

E.g.

type(...).configure({message: (ctx) => `no special characters (was ${ctx.actual})`})

type(...).configure({message: (ctx) => `no special characters (was ${ctx.actual})`})

cowboydOP•6/12/25, 1:52 PM

Ok, I can give that a shot. Cheers

cowboydOP•6/12/25, 2:06 PM

Also, not really clear how I configure just the index type, and not the value type.

cowboydOP•6/12/25, 2:15 PM

@TizzySaurus I can't seem to get this to work in the playground https://arktype.io/playground?code=import%2520%257B%2520type%2520%257D%2520from%2520%2522arktype%2522%250A%250Aconst%2520Thing%2520%253D%2520type%28%257B%250A%2509%27%252F%255Ba-z%255D%252B%252F%27%253A%2520%2522string%2522%252C%250A%257D%29%250A%250Aconst%2520out%2520%253D%2520Thing%28%257B%2522username%2522%253A%2520%2522bob%2522%257D%29%250A

ArkType

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Ccowboyd Also, not really clear how I configure _just_ the index type, and not the value ...

TizzySaurus•6/12/25, 3:28 PM

If you look at the docs you can override specific error codes, I.e. figure out the error code for violating the key and overwrite its message

Ccowboyd @TizzySaurus I can't seem to get this to work in the playground https://arktype....

TizzySaurus•6/12/25, 3:30 PM

I guess I've forgotten how to do index signatures correctly. You can scan through the docs / unit tests to figure that out

TizzySaurus•6/12/25, 3:32 PM

https://arktype.io/docs/objects#properties-index

Looks like it might be

{"[/[a-z]+/]": "string"}

{"[/[a-z]+/]": "string"}

ArkType

ArkType Docs

TypeScript's 1:1 validator, optimized from editor to runtime

Defining Records with narrowed keys

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