Accurate interval

AccurateInterval.js

function AccurateInterval(intervalMs, callback) {

    var repeat;
    var expected;
    var timer;

    function step() {
        var err = Date.now() - expected;
        callback();
        console.log("error was", err, "ms");
        if (repeat) {
            expected += intervalMs;
            timer = setTimeout(step, Math.max(0, intervalMs - err));
        }
    }

    return {
        start: function () {
            repeat = true;
            expected = Date.now() + intervalMs;
            timer = setTimeout(step, intervalMs);
        },
        stop: function () {
            repeat = false;
            clearTimeout(timer);
        }
    };
}