Map Sum Pairs LeetCode solution in JavaScript

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var MapSum = function () {
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  this.hashKeys = new Map();
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  this.hashVals = new Map();
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};
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MapSum.prototype.insert = function (key, val) {
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  let prefix = "";
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  for (const str of key) {
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    prefix += str;
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    const hashKey = this.hashKeys.get(prefix) ?? new Set();
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    hashKey.add(key);
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    !this.hashKeys.has(prefix) && this.hashKeys.set(prefix, hashKey);
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  }
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  this.hashVals.set(key, val);
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};
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MapSum.prototype.sum = function (prefix) {
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  const hashKey = this.hashKeys.get(prefix);
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  if (!hashKey) return 0;
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  let sum = 0;
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  hashKey.forEach((key) => (sum += this.hashVals.get(key)));
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  return sum;
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};