Stopwatch in JavaScript

1class Stopwatch {
2    constructor() { this.startTime = null; this.elapsed = 0; }
3    start() { this.startTime = Date.now() - this.elapsed; }
4    stop() { if(this.startTime) this.elapsed = Date.now() - this.startTime; }
5    reset() { this.elapsed = 0; this.startTime = null; }
6    getTime() { return this.elapsed; }
7}