Find busiest period in building
From a short review;


I advise avoiding null, you could either not define the member or use undefined
busiestPeriod is a really long name, given how often you use it
You really only need to track the log entry with the most visitors, the start of the period would be timestamp of the entry before that
This: busiestPeriod.visitors = v.type === "enter" ?
  busiestPeriod.visitors + v.count :
  busiestPeriod.visitors - v.count; could be 

busiestPeriod.visitors += (v.type == "enter" ? v.count : - v.count); 

or even 

busiestPeriod.visitors += (v.type == "enter" ? 1 : -1) * v.count;


This is my counter-proposal:

function findBusiestTime(logs){

  function analyzeLogEntry(acc, log, index){
    if(log.type == "enter"){
      acc.count += log.count;   
    }else{
      if(acc.count &gt; acc.maxCount){
        acc.maxCount = acc.count;
        acc.index = index;
      }
      acc.count -= log.count;
    }
    return acc;
  }

  let {index} = logs.reduce(analyzeLogEntry,{count:0, maxCount: 0});
  return [logs[index-1].timestamp,logs[index].timestamp];  
}