Timer for count-down and count-up

Timer.js

/**
 * Timer class for count-down and count-up
 */
class Timer {
    constructor (duration = 0, countDown = true, step = 1) {
        this.duration = duration;
        this.countDown = countDown;
        this.step = step;
        this.opr = countDown ? -1 : 1;
        this.currentTime = this.freshCurrentTime;
        this.id = null;
    }

    /**
     * get fresh current time
     * @returns {number}
     */
    get freshCurrentTime () {
        return this.countDown ? this.duration : 0;
    }

    /**
     * tick timer for one step
     */
    tick () {
        this.currentTime = this.currentTime + this.opr * this.step;
    }

    /**
     * start timer count
     */
    start () {
        this.id = setInterval(() => {
            if (
                (this.countDown && this.currentTime > 0) ||
                (!this.countDown && this.currentTime < this.duration)
            ) {
                this.tick();
            } else {
                this.stop();
            }
        }, this.step * 1000);
        this.tick();
    }

    /**
     * stop timer count
     */
    stop () {
        clearInterval(this.id);
        this.id = null;
    }

    /**
     * reset timer properties
     */
    reset () {
        this.stop();
        this.currentTime = this.freshCurrentTime;
    }

    /**
     * get current time
     * @returns {number|*}
     */
    get second () {
        return this.currentTime;
    }
}

What is duration used for?

 reset () {
        this.stop();
        this.currentTime = this.freshCurrentTime;
        this.id = null;
    }

I think it's better to move this.id = null to stop method

if (this.countDown && this.currentTime > 0) {
                this.tick();
            } else if (!this.countDown && this.currentTime < this.duration) {
                this.tick();
            } else {
                this.stop();
            }```

You can combine "if" and "else if" conditions with an "or" operator (||)