Possible solution to https://www.hackerrank.com/challenges/sock-merchant/problem

sales-by-match.js

var n = 9;
var ar = [10, 20, 20, 10, 10, 30, 50, 10, 20]

function sockMerchant(n, arr) {
    const hash = new Map();
    let count = 0;

    for(let i = 0; i < n; i++) {
        const sock = arr[i];
        if (hash.get(sock) == 1) {
            hash.set(sock, 0);
            count += 1;
        } else {
            hash.set(sock, 1);
        }
    }

    return count;
}

console.log(sockMerchant(n, ar));