Design Underground System LeetCode solution in JavaScript

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var UndergroundSystem = function () {
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  this.checkInMap = {};
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  this.tripMap = {};
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};
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/**
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 * @param {number} id
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 * @param {string} stationName
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 * @param {number} t
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 * @return {void}
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 */
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UndergroundSystem.prototype.checkIn = function (id, stationName, t) {
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  this.checkInMap[id] = [stationName, t];
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};
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/**
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 * @param {number} id
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 * @param {string} stationName
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 * @param {number} t
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 * @return {void}
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 */
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UndergroundSystem.prototype.checkOut = function (id, stationName, t) {
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  const [checkInStationName, checkInTime] = this.checkInMap[id];
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  const tripKey = `${checkInStationName}-${stationName}`;
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  if (!this.tripMap[tripKey]) this.tripMap[tripKey] = [0, 0];
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  this.tripMap[tripKey][0] += t - checkInTime;
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  this.tripMap[tripKey][1]++;
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};
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/**
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 * @param {string} startStation
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 * @param {string} endStation
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 * @return {number}
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 */
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UndergroundSystem.prototype.getAverageTime = function (
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  startStation,
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  endStation
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) {
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  const tripkey = `${startStation}-${endStation}`;
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  const [totalTripTime, totalTrips] = this.tripMap[tripkey];
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  return totalTripTime / totalTrips;
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};