Most Frequent Number Following Key In an Array LeetCode solution in JavaScript

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/**
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 * @param {number[]} nums
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 * @param {number} key
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 * @return {number}
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 */
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var mostFrequent = function (nums, key) {
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  const obj = {}; // to keep track the occurances after the key
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  let count = 0;
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  let res = nums[0];
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  for (let i = 1; i < nums.length; i++) {
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    if (nums[i - 1] === key) {
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      if (obj[nums[i]]) {
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        obj[nums[i]] += 1;
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      } else {
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        obj[nums[i]] = 1;
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      }
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      if (count < obj[nums[i]]) {
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        count = obj[nums[i]];
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        res = nums[i];
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      }
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    }
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  }
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  return res;
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};