Cinema Seat Allocation LeetCode solution in JavaScript

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/**
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 * @param {number} n
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 * @param {number[][]} reservedSeats
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 * @return {number}
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 */
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var maxNumberOfFamilies = function (n, reservedSeats) {
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  let a,
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    b,
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    c,
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    reservedMap = {},
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    ans = n * 2,
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    takenB = {};
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  for (let i = 0; i < reservedSeats.length; i++) {
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    let resverd = reservedSeats[i];
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    let row = resverd[0];
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    if (reservedMap[row] === undefined) {
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      reservedMap[row] = {};
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    }
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    if (
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      2 <= resverd[1] &&
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      resverd[1] <= 3 &&
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      reservedMap[row]["a"] === undefined
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    ) {
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      //a is blocked now
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      ans--;
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      reservedMap[row]["a"] = true;
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      if (
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        reservedMap[row]["c"] === true &&
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        reservedMap[row]["b"] === undefined
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      ) {
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        takenB[resverd[0]] = true;
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        ans++; //add for b
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      }
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    } else if (
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      8 <= resverd[1] &&
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      resverd[1] <= 9 &&
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      reservedMap[row]["c"] === undefined
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    ) {
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      //c is blocked now
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      ans--;
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      reservedMap[row]["c"] = true;
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      if (
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        reservedMap[row]["a"] === true &&
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        reservedMap[row]["b"] === undefined
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      ) {
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        takenB[resverd[0]] = true;
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        ans++; //add for b
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      }
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    } else if (4 <= resverd[1] && resverd[1] <= 5) {
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      //b is blocked now
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      if (reservedMap[row]["a"] === undefined) {
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        ans--;
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        reservedMap[row]["a"] = true;
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      }
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      if (takenB[resverd[0]] === true) {
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        ans--; //substract for b
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        takenB[resverd[0]] = false;
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      }
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      reservedMap[row]["b"] = true;
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    } else if (6 <= resverd[1] && resverd[1] <= 7) {
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      //b is blocked now
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      if (reservedMap[row]["c"] === undefined) {
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        ans--;
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        reservedMap[row]["c"] = true;
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      }
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      if (takenB[resverd[0]] === true) {
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        ans--; //substract for b
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        takenB[resverd[0]] = false;
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      }
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      reservedMap[row]["b"] = true;
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    }
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  }
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  return ans;
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};