function singularRelativeTimeUnit(unit) {
        invariant(typeof unit === 'string', "unit must be a string, instead got " + typeof unit, TypeError);
        if (unit === 'seconds')
            return 'second';
        if (unit === 'minutes')
            return 'minute';
        if (unit === 'hours')
            return 'hour';
        if (unit === 'days')
            return 'day';
        if (unit === 'weeks')
            return 'week';
        if (unit === 'months')
            return 'month';
        if (unit === 'quarters')
            return 'quarter';
        if (unit === 'years')
            return 'year';
        if (unit !== 'second' &&
            unit !== 'minute' &&
            unit !== 'hour' &&
            unit !== 'day' &&
            unit !== 'week' &&
            unit !== 'month' &&
            unit !== 'quarter' &&
            unit !== 'year') {
            throw new RangeError("Invalid unit " + unit);
        }
        return unit;
    }
https://tc39.es/proposal-intl-relative-time/#sec-singularrelativetimeunit
@param unit