You're right, my implementation was not a drop-in replacement for replace. How about this?

    String.prototype.replaceAll = function (n, sub) {
      let t, f = "g";
      if (n instanceof RegExp) {
        t = n.source;
        f = n.flags;
        if (!f.includes("g"))
          f += "g";
      } else {
        t = RegExp.escape(n);
      }
      let r = new RegExp(t, f);
      return this.replace(r, sub || "");
    }