Count pairs of socks in a merchant array - JS functional programming using variable closure

sockMerchant.js

// run the code at: https://jsfiddle.net/shayan_ys/5qos4agz/
// question found on HackerRank: https://www.hackerrank.com/challenges/sock-merchant?h_l=interview&playlist_slugs%5B%5D=interview-preparation-kit&playlist_slugs%5B%5D=warmup

function isPair(x, pairs) {
    return pairs.hasOwnProperty(x);
}

var processMerchant = (function () {
    var pairs = {};
    var pairsCount = 0;

    function processValue(value) {
        if (isPair(value, pairs)) {
            delete pairs[value];
            pairsCount++;
        } else {
            pairs[value] = true;
        }
    }

    function countPairs(n, ar) {
        ar.map((i) => {
            processValue(i);
            return i;
        });

        return pairsCount;
    }

    return countPairs;
})();

function sockMerchant(n, ar) {
    return processMerchant(n, ar);
}

var init = (function() {
    var n = 9;
    var ar = [10, 20, 20, 10, 10, 30, 50, 10, 20];
  
    let pairs = sockMerchant(n, ar);
    console.log(pairs);
    document.getElementById("result").innerText = pairs.toString();
})();

Run the code at: jsfiddle.net/shayan_ys/5qos4agz
The question was found on HackerRank