src/lib/es2018.intl.d.ts

declare namespace Intl {

    // http://cldr.unicode.org/index/cldr-spec/plural-rules#TOC-Determining-Plural-Categories
    type LDMLPluralRule = "zero" | "one" | "two" | "few" | "many" | "other";
    type PluralRuleType = "cardinal" | "ordinal";

    interface PluralRulesOptions {
        localeMatcher?: "lookup" | "best fit";
        type?: PluralRuleType;
        minimumIntegerDigits?: number;
        minimumFractionDigits?: number;
        maximumFractionDigits?: number;
        minimumSignificantDigits?: number;
        maximumSignificantDigits?: number;
    }

    interface ResolvedPluralRulesOptions {
        locale: string;
        pluralCategories: LDMLPluralRule[];
        type: PluralRuleType;
        minimumIntegerDigits: number;
        minimumFractionDigits: number;
        maximumFractionDigits: number;
        minimumSignificantDigits?: number;
        maximumSignificantDigits?: number;
    }

    interface PluralRules {
        resolvedOptions(): ResolvedPluralRulesOptions;
        select(n: number): LDMLPluralRule;
    }

    const PluralRules: {
        new (locales?: string | string[], options?: PluralRulesOptions): PluralRules;
        (locales?: string | string[], options?: PluralRulesOptions): PluralRules;
        supportedLocalesOf(
            locales: string | string[],
            options?: PluralRulesOptions,
        ): string[];
    };
}