TypeScript restriction using a const stop working when typing the const itself

Question

Using TypeScript 4.6.4, I have a list of keywords and a Paragraph interface where these keywords are referenced:

const keyswordsAndSynonyms = {
    keyword1: ["synonym1", "synonym2"],
    keyword2: ["synonym3", "synonym4"],
} as const

interface Paragraph {
  content: string
  keywords: Array<keyof typeof keyswordsAndSynonyms>
}

const myParagraph: Paragraph = {
  content: "bla bla",
  keywords: ["keyword1", "foo dahoo"]
}

So far, so good. TypeScript complains that "foo dahoo" is wrong - as it should.

Now I want to type keyswordsAndSynonyms itself. (Just to be save so that no one can accidentally use a flat string as synonym or a number as keyword or whatnot.)

However, when typing keyswordsAndSynonyms as Record<string, readonly string[]>, the type restrictions for keywords stop working. I can now use "foo dahoo" without TypeScript complaining about it.

How come and how can I solve that?

Why do you want to use `as const` at all, since you want to use `Record` ? — captain-yossarian from Ukraine, May 24 '22 at 06:42
@captain-yossarian I wondered about that too, but I think the goal is to put an additional restriction in place that prevents other developers from corrupting the structure of that `const` value. — Robby Cornelissen, May 24 '22 at 06:49
AFAIK, TS does not support both `as const` assertion and explicit type. In order to do more advanced validation you should use function and infer provided argument. For more context see [my article](https://catchts.com/validators) — captain-yossarian from Ukraine, May 24 '22 at 06:52

TypeScript restriction using a const stop working when typing the const itself

0 Answers0