index.js

const { writeFileSync } = require("node:fs");

process.env['NODE_TLS_REJECT_UNAUTHORIZED'] = '0';

class EternalReturn {

  #_baseURL = 'https://bser-rest-release.bser.io/api';
  #_gameVersion = '1.22.0';
  #_replayVersion = '1.22.0';

  constructor(token) {
    this._token = token.replace('Session:', '');
    this._headers = {
      method: 'GET',
      headers: {
        'X-BSER-SessionKey': `Session:${this._token}`,
        'X-BSER-Version': this.#_gameVersion,
        'X-BSER-Replay-Version': this.#_replayVersion,
        'X-BSER-AuthProvider': 'STEAM',
        'Content-Type': 'application/json'
      }
    }
  }

  async getReplay(gameId) {
    try {
      if (!gameId.trim()) throw new Exceptions(0);

      const response = await fetch(`${this.#_baseURL}/external/findReplayGame/${gameId}/0`, this._headers);
      const data = await response.json();

      if (data.cod !== 200) throw new Exceptions(data.cod);

      await this.#downloadReplay(data.rst.replayPath, gameId);
    } catch (e) {
      console.error(e)
    }
  }

  async #downloadReplay(url, gameId) {
    const format = url.split(".").pop();
    const response = await fetch(url);
    const replay = await response.text();
    writeFileSync(`./EternalReturn-${gameId}.${format}`, replay);
  }
}

class Exceptions extends Error {

  #errors = {
    0: 'Missing gameId',
    1001: 'Parameter type mismatch',
    1007: 'Invalid Version (Game)',
    1102: 'Invalid Token',
    1201: 'Not exist this gameId',
    9300: 'Invalid ID Replay version'
  }

  constructor(cod) {
    super()
    this.name = "Eternal Return Exception"
    this.message = this.#errors[cod]
  }
}

(async () => {
  const er = new EternalReturn('<Token>');
  await er.getReplay('<gameID>');
})();