Challenge 3 — Solution

Task: Create an object stopwatch with elapsed: 0 and a method
start() that uses setInterval with an ARROW function to increment
elapsed and log it every second, for 3 seconds (then stop with
clearInterval). Confirm elapsed actually increases by logging it.

const stopwatch = {
    elapsed: 0,
    start() {
        const intervalId = setInterval(() => {
            this.elapsed++;
            console.log(this.elapsed);

            if (this.elapsed >= 3) {
                clearInterval(intervalId);
            }
        }, 1000);
    }
};

stopwatch.start();


Expected output (one line per second, over 3 seconds):

1
2
3


Notes:
- The arrow function passed to setInterval has no this of its own,
  so it inherits this from start() — which is stopwatch, since
  start() was called as stopwatch.start(). Using a regular function
  here instead would have made this undefined inside the interval
  callback, breaking this.elapsed entirely.
- intervalId is captured by the same closure (Chapter 2) that the
  arrow function uses, which is how clearInterval(intervalId) can
  stop the very interval the callback is running inside.
- elapsed is checked AFTER incrementing it, so the interval fires
  exactly 3 times before clearing itself.