Jewels and Stones LeetCode solution in JavaScript

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var numJewelsInStones = function (jewels, stones) {
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  let trackDict = new Map();
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  let output = 0;
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  for (let char of stones)
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    trackDict.set(char, trackDict.has(char) ? trackDict.get(char) + 1 : 1);
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  for (let char of jewels)
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    output += trackDict.has(char) ? trackDict.get(char) : 0;
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  return output;
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};