Typescript: get exact type of variable

Question

Having an object like

const translations = {
  say_hello: 'hello {{name}}',
};

I'd like to extract the exact type of the object, like

type Translations = {
  say_hello: 'hello {{name}}',
}

So far I'm playing around something like

type GetExactType<T> = {[K in keyof T]-?: T[K]};

but it doesn't work as expected, because

  GetExactType<typeof translations>;

resolves to

{
   say_hello: string,
}

Any ideas?

You can't do it after that initialization of translations; the information you want has already been discarded. You can change the type of the initializer via a const assertion like this but I don't know if that meets your needs. — jcalz
– jcalz, Commented Sep 16, 2022 at 16:12
You want GetExactType<typeof translations> to return Translations? — mwilson
– mwilson, Commented Sep 16, 2022 at 16:31

k.tten · Accepted Answer · 2022-09-16 16:12:03Z

1

You could use as const but that results in readonly on everything, which might not be desirable.

Instead, you could have a function to narrow it for you:

type Narrow<T> =
    | (T extends infer U ? U : never)
    | Extract<T, number | string | boolean | bigint | symbol | null | undefined | []>
    | ([T] extends [[]] ? [] : { [K in keyof T]: Narrow<T[K]> });

function narrow<T>(t: Narrow<T>): T {
    return t as T;
}

And then when you use it, it'll infer the types for you:

const translations = narrow({
  say_hello: 'hello {{name}}',
});

type T = typeof translations;
//   ^? { say_hello: 'hello {{name}}'; }

Note that it is impossible to retrieve the original type after you define it like this:

const translations = {
  say_hello: 'hello {{name}}',
};

so some sort of direct operation on the value before it is assigned is required.

Playground

answered Sep 16, 2022 at 16:12

k.tten

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1 Comment

T.J. Crowder Over a year ago

Fascinating. I suspect, given this is what appears to be translation strings, the as const would be sufficient. But really cool to have that Narrow type in the toolkit...

T.J. Crowder · Accepted Answer · 2022-09-16 16:17:29Z

You have to tell TypeScript that the say_hello property's type is 'hello {{name}}', not string. There are at least a couple of ways to do that:

Via as const, saying that the object won't change:

const translations = {
    say_hello: 'hello {{name}}',
} as const;
//^^^^^^^^
type Translations = typeof translations;

Explicitly:

const translations = {
    say_hello: 'hello {{name}}' as 'hello {{name}}',
// −−−−−−−−−−−−−−−−−−−−−−−−−−−−^^^^^^^^^^^^^^^^^^^^
};
type Translations = typeof translations;

(but...ewww...)

or

const translations: { say_hello: 'hello {{name}}' } = {
    say_hello: 'hello {{name}}',
};
type Translations = typeof translations;
// (But if you were going to do that, you'd just want to write the type
// alias manually and then apply it to the `translations` constant, rather
// than doing it this way around.)

I use the as const for things like translation strings all the time.

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