Sock Merchant Hacker Rank

SockMerchant.js

function sockMerchant(n, ar) {
    const pairs={};
    return ar.reduce((memo,value)=>{
        if(pairs.hasOwnProperty(value))
        {
            pairs[value]=pairs[value]+1;
            memo+=pairs[value]%2===0?1:0
        }else{
            pairs[value]=1
        }
        return memo;
    },0)

}