How to ensure TypeScript that string|string[] is string without using as?

Question

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Answer 1

Besides generics you can use function overloading

function getI18n(id: string[]): string[];
function getI18n(id: string): string;
function getI18n(id: string | string[]): string | string[] {
    if (typeof id === 'string') {
        return id + '_title';
    }
    return id.slice();
}

const title = getI18n('test'); // const title: string
const titles = getI18n(['a', 'b', 'c']); // const titles: string[]

Link to official docs on this feature: functions overloads