code: function concat(a, b) {
    if (!a && !b) throw new Error("Please specify valid arguments for parameters a and b.");
    if (!b || b.length === 0) return a;
    if (!a || a.length === 0) return b;
    const c = new a.constructor(a.length + b.length);
    c.set(a);
    c.set(b, a.length);
    return c;
  }
docs: concat 2 typed array