Adds an ordinal suffix to a number. Use the modulo operator (%) to find values of single and tens digits. Find which ordinal pattern digits match. If digit is found in teens pattern, use teens ordinal.

/**
 * Adds an ordinal suffix to a number.
 *
 * Use the modulo operator (%) to find values of single and tens digits. Find
 * which ordinal pattern digits match. If digit is found in teens pattern, use
 * teens ordinal.
 *
 * @param {[type]} num
 * @return {[type]} [description]
 */
const toOrdinalSuffix = num => {
    const int = parseInt(num),
        digits = [int % 10, int % 100],
        ordinals = ["st", "nd", "rd", "th"],
        oPattern = [1, 2, 3, 4],
        tPattern = [11, 12, 13, 14, 15, 16, 17, 18, 19];
    return oPattern.includes(digits[0]) && !tPattern.includes(digits[1])
        ? int + ordinals[digits[0] - 1]
        : int + ordinals[3];
};

//
// Example Usage:
toOrdinalSuffix("123"); // "123rd"