Countdown Timer in JavaScript

1class Countdown {
2    constructor(target) { this.target = target; }
3    start(callback) {
4        const interval = setInterval(() => {
5            const remaining = this.target - Date.now();
6            if(remaining <= 0) { clearInterval(interval); callback(); }
7            else callback(remaining);
8        }, 1000);
9    }
10}