Number of Sub-arrays of Size K and Average Greater than or Equal to Threshold LeetCode solution in JavaScript

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var numOfSubarrays = function (arr, k, threshold) {
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  let total = 0;
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  let left = 0;
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  let right = k;
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  // get initial sum of numbers in first sub array range, by summing left -> right
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  let sum = arr.slice(left, right).reduce((a, b) => a + b, 0);
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  while (right <= arr.length) {
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    // move through the array as a sliding window
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    if (left > 0) {
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      // on each iteration of the loop, subtract the val that has fallen out of the window,
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      // and add the new value that has entered the window
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      sum -= arr[left - 1];
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      sum += arr[right - 1];
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    }
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    if (sum / k >= threshold) {
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      total++;
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    }
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    left++;
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    right++;
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  }
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  return total;
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};