Bidders.js

'use strict';

class Bidder {
    constructor(quantity, cash) {
        /*
         quantity: integer even number of lots
                   each time exactly 2 lots are sold
                   so there will be exactly quantity/2 iterations
                   it's guaranteed that this number is positive

         cash: integer number, amount of cents
               it's guaranteed that this number is positive
         */
    }

    placeBid() {
        /*
         returns the next bid for the product, which may be zero
         must be non-negative integer
         */
        return 0;
    }

    bids(own, other) {
        /*
         shows the bids of the two bidders
         will be called before each 'placeBid' call except the first one
         */
    }
}

function run(first_bidder_class, second_bidder_class, quantity, cash) {
    /*
    returns 1 or 2 depending on which bidder won
    returns 0 on draw
     */
    if (quantity % 2 !== 0) {
        throw `quantity should be even! got ${quantity} instead.`
    }

    let bidder1 = new first_bidder_class(quantity, cash),
        bidder2 = new second_bidder_class(quantity, cash),
        bid1,
        bid2,
        score1 = 0,
        score2 = 0,
        spent1 = 0,
        spent2 = 0;

    while (quantity > 0) {
        if (typeof bid1 !== 'undefined' && typeof bid2 !== 'undefined') {
            // call always except the 1st iteration
            bidder1.bids(bid1, bid2);
            bidder2.bids(bid2, bid1);
        }

        // make bids
        bid1 = parseInt(bidder1.placeBid());
        bid2 = parseInt(bidder2.placeBid());
        // validate
        if (Number.isNaN(bid1) || bid1 < 0) {
            throw 'First bidder has returned incorrect bid';
        }
        if (Number.isNaN(bid2) || bid2 < 0) {
            throw 'Second bidder has returned incorrect bid';
        }

        // validate total sum
        spent1 += bid1;
        spent2 += bid2;
        if (spent1 > cash) {
            throw 'First bidder has exceeded given cash amount';
        }
        if (spent2 > cash) {
            throw 'Second bidder has exceeded given cash amount';
        }

        // give scores
        if (bid1 > bid2) {
            score1 += 2;
        } else if (bid2 > bid1) {
            score2 += 2;
        } else {
            score1 += 1;
            score2 += 1;
        }

        quantity -= 2;
    }

    if (spent1 !== cash) {
        throw `First bidder has not spent all cash, remaining sum: ${cash - spent1}`;
    }

    if (spent2 !== cash) {
        throw `Second bidder has not spent all cash, remaining sum: ${cash - spent2}`;
    }

    return score1 > score2 ? 1 : score2 > score1 ? 2 : 0;
}

function runNTimes(N, first_bidder_class, second_bidder_class, quantity, cash) {
    let totalScore1 = 0,
        totalScore2 = 0,
        result;
    for (let i = 0; i < N; i++) {
        result = run(first_bidder_class, second_bidder_class, quantity, cash);
        if (result === 1) {
            totalScore1 += 1;
        } else if (result === 2) {
            totalScore2 += 1;
        }
    }
    console.log(`1st Bidder: ${totalScore1} (${N - totalScore2} with draws)`);
    console.log(`2nd Bidder: ${totalScore2} (${N - totalScore1} with draws)`);
    console.log(`     Draws: ${N - totalScore1 - totalScore2}`);
}