lib/auth/googleSACredential.ts

import type { JWTInput, RefreshOptions } from "google-auth-library";
import { GoogleAuth } from "google-auth-library";
import type { JSONClient } from "google-auth-library/build/src/auth/googleauth";
import { SCOPE } from "../common/constants";
import type { Credential } from "./interfaces";

export class GoogleSACredential implements Credential {
  private json: string | JWTInput;
  private options: RefreshOptions | undefined;
  protected auth?: JSONClient;

  /**
   *
   * @param json The input object.
   * @param options The JWT or UserRefresh options for the client
   */
  constructor(json: string | JWTInput, options?: RefreshOptions) {
    if (!json) {
      throw new Error("'json' must be set when using service account flow.");
    }

    this.json = json;
    this.options = options;

    this.build();
  }

  private async build() {
    const json =
      typeof this.json === "string" ? JSON.parse(this.json) : this.json;

    this.auth = new GoogleAuth({
      scopes: SCOPE,
    }).fromJSON(json, this.options);
  }

  async getToken(): Promise<string | null | undefined> {
    return (await this.auth?.getAccessToken())?.token;
  }
}