youtube-looper.js

class Looper {
  constructor(opts) {
    this.video = $("video");
    this.loop(opts);
  }

  loop(opts = {}) {
    clearTimeout(this.loopTimeoutId);
    this.setOpts(opts);
    this.video.playbackRate = this.playbackRate;
    this.video.currentTime = this.start;
    this.video.play();
    const duration = (this.end - this.start) * 1000 / this.playbackRate;
    this.loopTimeoutId = setTimeout(() => this.loop(), duration);
  }

  stop() {
    clearTimeout(this.loopTimeoutId);
    this.video.pause();
  }

  setOpts(opts) {
    const isNumber = (x) => typeof x === "number";

    if (isNumber(opts.start)) {
      this.start = opts.start;
    } else if (!isNumber(this.start)) {
      throw new Error('missing start');
    }

    if (isNumber(opts.end)) {
      this.end = opts.end;
    } else if (!isNumber(this.end)) {
      throw new Error('missing end');
    }

    if (isNumber(opts.playbackRate)) {
      this.playbackRate = opts.playbackRate;
    } else if (!isNumber(this.playbackRate)) {
      this.playbackRate = 1;
    }
  }
}

// example
const looper = new Looper({ start: 0, end: 17, playbackRate: 0.7 })