Check if All the Integers in a Range Are Covered LeetCode solution in JavaScript

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/**
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 * @param {number[][]} ranges
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 * @param {number} left
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 * @param {number} right
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 * @return {boolean}
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 */
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var isCovered = function (ranges, left, right) {
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  let z = 0,
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    f = 0,
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    e = [];
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  let a = ranges.flat(Infinity);
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  function abc(c, d) {
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    for (let i = c; i <= d; i++) if (e[i] == i) e[i] = null;
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  }
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  for (let i = left; i <= right; i++) e[i] = i;
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  for (let i = 0; i < a.length; i = i + 2) {
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    if (a[i] <= left && a[i + 1] >= right) return true;
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    else if (a[i] > right) continue;
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    else abc(a[i], a[i + 1]);
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  }
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  for (let i = left; i <= right; i++) if (e[i] != null) return false;
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  return true;
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};