nplural.js

function declension(amount, words) {
    var digit = (Number(amount) > 20) ? Number(amount) % 10 : Number(amount);

    if (digit >= 5 || digit == 0) {
        return words[2]
    }

    if (digit >= 2) {
        return words[1]
    }

    return words[0];
}

function plural(amount, words) {
    return amount + ' ' + declension(amount, words);
}