Number of Equivalent Domino Pairs LeetCode solution in JavaScript

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var numEquivDominoPairs = function (dominoes) {
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  let map = {};
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  let count = 0;
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  for (let [a, b] of dominoes) {
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    if (b > a) {
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      [a, b] = [b, a];
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    }
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    let key = `${a}-${b}`;
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    if (map.hasOwnProperty(key)) {
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      map[key]++;
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      count += map[key];
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    } else {
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      map[key] = 0;
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    }
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  }
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  return count;
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};