Cron expression parser with next run times

type Spec = { min: number; max: number };
const FIELDS: Spec[] = [{ min: 0, max: 59 }, { min: 0, max: 23 }, { min: 1, max: 31 }, { min: 1, max: 12 }, { min: 0, max: 7 }];

// One field -> lookup table indexed by value.
function parseField(src: string, { min, max }: Spec): boolean[] {
  const on = new Array<boolean>(max + 1).fill(false);
  for (const part of src.split(',')) {
    const [range = '', stepText] = part.split('/');
    const step = stepText === undefined ? 1 : Number(stepText);
    if (!Number.isInteger(step) || step < 1) throw new Error(`Bad step in "${part}"`);
    let from = min;
    let to = max;
    if (range !== '*') {
      const [a, b] = range.split('-');
      from = Number(a);
      to = b === undefined ? (stepText === undefined ? from : max) : Number(b);
    }
    if (!Number.isInteger(from) || !Number.isInteger(to) || from < min || to > max || from > to) {
      throw new Error(`Value out of range in "${part}"`);
    }
    for (let v = from; v <= to; v += step) on[v] = true;
  }
  return on;
}

export function parseCron(expr: string) {
  const parts = expr.trim().split(/\s+/);
  if (parts.length !== 5) throw new Error('Expected 5 fields: minute hour day month weekday');
  const [minute, hour, dom, month, dow] = parts.map((p, i) => parseField(p, FIELDS[i]!)) as boolean[][];
  if (dow![7]) dow![0] = true; // 7 is Sunday too
  return { minute: minute!, hour: hour!, dom: dom!, month: month!, dow: dow!, domAny: parts[2] === '*', dowAny: parts[4] === '*' };
}

export function nextRuns(expr: string, from = new Date(), count = 5): Date[] {
  const c = parseCron(expr);
  const out: Date[] = [];
  const d = new Date(from);
  d.setSeconds(0, 0);
  d.setMinutes(d.getMinutes() + 1);
  const limit = d.getTime() + 5 * 366 * 864e5;

  while (out.length < count && d.getTime() < limit) {
    const dayOk = c.domAny && c.dowAny ? true
      : c.domAny ? c.dow[d.getDay()]!
      : c.dowAny ? c.dom[d.getDate()]!
      : c.dom[d.getDate()]! || c.dow[d.getDay()]!; // both restricted: either matches
    if (!c.month[d.getMonth() + 1]) { d.setMonth(d.getMonth() + 1, 1); d.setHours(0, 0, 0, 0); continue; }
    if (!dayOk) { d.setDate(d.getDate() + 1); d.setHours(0, 0, 0, 0); continue; }
    if (!c.hour[d.getHours()]) { d.setHours(d.getHours() + 1, 0, 0, 0); continue; }
    if (!c.minute[d.getMinutes()]) { d.setMinutes(d.getMinutes() + 1); continue; }
    out.push(new Date(d));
    d.setMinutes(d.getMinutes() + 1);
  }
  return out;
}