JavaScript Global Variable Not Changing

Question

I'm new to JavaScript and I made a simple stopwatch constructor:

function Stopwatch() {
    var started = false;
    var elapsed;
    var start;
    d = new Date();
    this.duration = function() {
        if (started) {
            throw "Stopwatch is still running"
        } else {
            return (elapsed);
        }
    }
    this.start = function() {
        if (started) {
            throw "Stopwatch has already started"; 
        } else {
            start = d.getTime();
            started = true;
        }
    }
    this.stop = function() {
        if (!started) {
            throw "Stopwatch hasn't started";
        } else {
            elapsed = d.getTime() - start;
            started = false;
        }
    }
    this.reset = function() {
        started = false;
        elapsed = 0;
    }
}
let sw = new Stopwatch();
sw.start();

setTimeout(function () {
    sw.stop();
    console.log(sw.duration());
}, 500);

The logic of the errors works fine, but whenever I call sw.duration(), it always returns zero, and I'm not sure why that is since I'm using var instead of let. Please let me know.

You should check out ES6 classes, because this isn’t the ideal way to create a constructor. Also, no reason to use a mix of `var` and `let` – as long as you have `let` available, you should never use `var`. Finally, implicit globals are bad, so make sure you’re in strict mode. — Ry-, Nov 23 '21 at 02:41

score 4 · Accepted Answer · answered Nov 23 '21 at 02:42

You're calling new Date only once, at the start of the constructor. Use Date.now instead (to get a number, to avoid having to call .getTime), and call it not only when you need to assign to start, but also in .stop, to get elapsed.

function Stopwatch() {
    var started = false;
    var elapsed;
    var start;
    this.duration = function() {
        if (started) {
            throw "Stopwatch is still running"
        } else {
            return (elapsed);
        }
    }
    this.start = function() {
        if (started) {
            throw "Stopwatch has already started"; 
        } else {
            start = Date.now();
            started = true;
        }
    }
    this.stop = function() {
        if (!started) {
            throw "Stopwatch hasn't started";
        } else {
            elapsed = Date.now() - start;
            started = false;
        }
    }
    this.reset = function() {
        started = false;
        elapsed = 0;
    }
}
let sw = new Stopwatch();
sw.start();
setTimeout(() => {
  sw.stop();
  console.log(sw.duration());
}, 500);

JavaScript Global Variable Not Changing

1 Answers1