A method for handling specific subsets of an enum in a secure and controlled manner

I have an enumerated type called Country and another type that represents a subset of European countries. I want to handle values within this subset differently from others. Currently, I am using an if statement with multiple conditions, but it could get unwieldy as the number of values in the subset grows.

enum Country {
    Canada,
    Finland,
    France,
    Germany,
    Japan,
    Peru,
}

type EuropeanCountries = Country.Finland | Country.France | Country.Germany;

const NonEuropeanCountriesContinent: Record<
    Exclude<Country, EuropeanCountries>,
    string
> = {
    [Country.Canada]: "America",
    [Country.Japan]: "Asia",
    [Country.Peru]: "America",
}

function getContinent(country: Country) {
    // Instead of manually checking each value, is there a better way to determine if the country is in the subset?
    if (country === Country.France || country === Country.Finland || country === Country.Germany) {
        return "Europe";
    }

    return NonEuropeanCountriesContinent[country];
}

How can I efficiently verify whether the value of my country variable is among the specified European countries without having to individually inspect each one?

Playground link

typescriptenums

Answer №1

If you want to keep track of which enum values are part of the EU and which are not, you can create a Custom Type Guard to verify the type in the following way:

enum Country {
    Spain,
    Italy,
    Greece,
    Netherlands,
    China,
    Brazil,
}

type EuropeanCountries = Country.Spain | Country.Italy | Country.Netherlands;
const EuropeContinent: Record<EuropeanCountries, string> = {
    [Country.Spain]: "Europe",
    [Country.Italy]: "Europe",
    [Country.Netherlands]: "Europe",
}
const NonEuropeanCountriesContinent: Record<
    Exclude<Country, EuropeanCountries>,
    string
> = {
    [Country.Greece]: "Europe",
    [Country.China]: "Asia",
    [Country.Brazil]: "America",
}

function getContinent(country: Country) {
    if(isInEurope(country)) {
        return EuropeContinent[country];
    }

    return NonEuropeanCountriesContinent[country];
}

function isInEurope(country: EuropeanCountries | Exclude<Country, EuropeanCountries>): country is EuropeanCountries {
    return !!EuropeContinent[country as EuropeanCountries];
}

console.log(getContinent(Country.Spain));
console.log(getContinent(Country.China));

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