Stopwatch.js

var startTime, lapTimes, endTime;

function start() {
  startTime = new Date();
  lapTimes = [];
};

function getPreviousTime(){
	let laps = lapTimes.length;
	return laps > 1 ? lapTimes[laps-2] : startTime;
}

function lap(message){
	let currentTime = new Date();
	lapTimes.push(currentTime);
	let timeDiff = currentTime - getPreviousTime();
	message = `(${message})`;
	console.log(`Lapped ${timeDiff}ms ${message}`);
}

function end() {
  endTime = new Date();
  if(lapTimes.length){
	let timeDiff = endTime - getPreviousTime();
	console.log(`Final lap: ${timeDiff}ms`);
  }
  let totalElapsed = endTime - startTime;
  console.log(`Total Elapsed: ${totalElapsed}ms`);
  reset();
}

function reset(){
	delete startTime;
	delete lapTimes;
	delete endTime;
}