Number of Equivalent Domino Pairs

LeetCode 題目: 1128. Number of Equivalent Domino Pairs

My solution:

/**
 * @param {number[][]} dominoes
 * @return {number}
 */
let numEquivDominoPairs = dominoes => {
    let pairsNumber = 0, map = new Map();
    for(const [a, b] of dominoes){
        let key = [Math.min(a,b), Math.max(a,b)].join();
        if(map.has(key)) {
            pairsNumber += map.get(key);
            map.set(key, map.get(key)+1);
        } else {
            map.set(key, 1);   
        }
    }   
    return pairsNumber;
};