function toJson(data) {

    return JSON.stringify(data, (_, v) => typeof v === 'bigint' ? `${v}n` : v)

        .replace(/"(-?\d+)n"/g, (_, a) => a);

}

​

function toJson(data) {

  if (data !== undefined) {

    let intCount = 0, repCount = 0;

    const json = JSON.stringify(data, (_, v) => {

      if (typeof v === 'bigint') {

        intCount++;

        return `${v}#bigint`;

      }

      return v;

    });

    const res = json.replace(/"(-?\d+)#bigint"/g, (_, a) => {

      repCount++;

      return a;

    });

    if (repCount > intCount) {

      // You have a string somewhere that looks like "123#bigint";

      throw new Error(`BigInt serialization conflict with a string value.`);

    }

    return res;

  }

}