Define a type for strings that are keys in an object

October 6, 2024

I have a country configuration which looks as such:

type CountryConfig = {
   [country: string]: {
      countryCode: string,
      callingCode: string,
      ...
   }
}

const balkanCountries: CountryConfig = {
   Albania: { countryCode: "AL", callingCode: "+355", ... },
   Bulgaria: { countryCode: "BG", callingCode: "+359", ... },
   Croatia: { countryCode: "HR", callingCode: "+385", ... },
   ...
}

I want to create a type from the keys of balkanCountries:

type BalkanCountry = keyof typeof balkanCountries

Expected type: BalkanCountry = "Albania" | "Bulgaria" | "Croatia" | ..., actual type: BalkanCountry = string | number.

Based on the answer in this Stack Overflow thread this is expected behaviour and I can get around the number type by explicitly extracting the string as such:

type BalkanCountry = Extract<keyof typeof balkanCountries, string>

However, the output type I get is still type BalkanCountries = string and not a union of literals.

This means I can do assignment as:

const currentCountry: BalkanCountry = "Spain" // typescript will not throw error

Why this is happening? How can I ensure currentCountry can only be a value that is a key in balkanCountries?

Note: I am using TypeScript ~5.0.4.

>Solution :

You will have to drop the type definition to infer the keys from the object.

const balkanCountries = {
   Albania: { countryCode: "AL", callingCode: "+355" },
   Bulgaria: { countryCode: "BG", callingCode: "+359",  },
   Croatia: { countryCode: "HR", callingCode: "+385",  },
} satisifies CountryConfig;

type BalkanCountry = keyof typeof balkanCountries
// "Albania" | "Bulgaria" | "Croatia"

The satisfies operator (introduced in TS 4.9), will ensure the type safety.

TS playground