A little subtitle file tweaking class

srt.js

class SRT {
  constructor(s) { this.setSubs(typeof s === 'string' ? SRT.decode(s) : s); }
  toString() { return SRT.encode(this.subs); }

  timeShift(dt) {
    return new SRT(this.subs.map(s => Object.assign({}, s, {t: s.t + dt})));
  }

  getSubs() { return this.subs.map(o => Object.assign({}, o)); }
  setSubs(subs) { this.subs = subs; return this; }

  static decode(s) { return s.split('\n\n').filter(i=>i).map(SRT.decodeItem); }
  static encode(subs) { return subs.map(SRT.encodeItem).join('\n\n') + '\n'; }

  static decodeTime(t) {
    return t.split(/[:,]/).reduce(
      (s, n, i) => parseInt(n, 10) + s * (i === 3 ? 1000 : 60), 0
    );
  }
  static encodeTime(n) {
    const fmt = (n, z) => (z + n).slice(-z.length);
    return [1000, 60, 60, Infinity].reduce((r, f, i) => {
      const t = n % f, sep = [new String(''), ','][i] || ':';
      n = (n - t) / f;
      return fmt(t, i ? '00' : '000') + sep + r;
    }, '');
  }

  static decodeItem(raw) {
    const s = raw.split('\n');
    const n = Number(s.shift());
    const [t, T] = s.shift().split(' --> ').map(SRT.decodeTime);
    const d = T - t;
    const m = s.join('\n');
    return { n, t, d, m };
  }
  static encodeItem({ n, t, d, m }) {
    return [ n
    , SRT.encodeTime(t) + ' --> ' + SRT.encodeTime(t + d)
    , m
    ].join('\n');
  }
}