Count Unhappy Friends LeetCode solution in JavaScript

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var unhappyFriends = function (n, preferences, pairs) {
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  let happyMap = new Array(n);
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  for (let [i, j] of pairs) {
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    happyMap[i] = preferences[i].indexOf(j);
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    happyMap[j] = preferences[j].indexOf(i);
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  }
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  let unhappy = 0;
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  for (let i = 0; i < n; i++) {
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    for (let j = 0; j < happyMap[i]; j++) {
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      let partner = preferences[i][j];
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      if (preferences[partner].indexOf(i) < happyMap[partner]) {
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        unhappy++;
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        break;
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      }
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    }
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  }
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  return unhappy;
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};