Евгений Бучинский

class Player{
    constructor(elo){
        this.elo = elo;
    }
}

let reducer = (accumulator, value) => accumulator + value.elo;

let players = [
    new Player(3000),
    new Player(700),
    new Player(750),
    new Player(1250),
    new Player(1000),
    new Player(1200),
    new Player(1350),
    new Player(1400),
    new Player(1500),
    new Player(745)
];

let combinations = (function () {

    function combinations(arr, k, start, idx, current, callback) {
        if (idx === k)
            return callback(current);

        for(let i = start; i < arr.length; i++) {
            current[idx] = arr[i];
            combinations(arr, k, i + 1, idx + 1, current, callback);
        }
    }

    return function (arr, k, callback) {
        combinations(arr, k, 0, 0, [], callback);
    };
}());

let minimal = [];

combinations(players, players.length / 2, (combo) => {

    let t1 = [...combo],
        t2 = players.filter(p => t1.indexOf(p) < 0),
        w1 = t1.reduce(reducer, 0),
        w2 = t2.reduce(reducer, 0),
        diff = Math.abs(w1 - w2);

    if( ! minimal.length)
        minimal = [t1, t2, diff];

    if(minimal[2] > diff)
        minimal = [t1, t2, diff];
});

console.log(minimal);