Example of strongly typed appconfig validation with simple array of keys

strongly-typed-array-keys-appconfig.ts

const configKeys = [
  "NODE_ENV",
  "PORT",
  "SECRET1",
  "SECRET2",
  "SECRET3",
  "ASYNC_SECRET",
] as const;

export type AppConfig = {
  [key in typeof configKeys[number]]: string;
};

export async function getAppConfig(): Promise<AppConfig> {
  /**
   * Load any async secrets separately separately
   */
  const [secret1, asyncSecret] = await Promise.all([
    getMockAsyncSecret(),
    getMockAsyncSecret(),
  ]);

  const config: Partial<AppConfig> = {
    SECRET1: secret1,
    ASYNC_SECRET: asyncSecret,
  };

  return configKeys.reduce<AppConfig>((appConfig, key) => {
    const configValue = process.env[key];

    if (appConfig[key]) {
      // we already have it from the preloaded secrets
      return appConfig;
    }

    if (!configValue) {
      // we should have it and we don't, throw an error
      throw new Error(`Missing config value for key: ${key}`);
    }

    appConfig[key] = configValue;

    return appConfig;
  }, config as AppConfig);
}

async function getMockAsyncSecret(): Promise<string> {
  return new Promise((resolve) =>
    setTimeout(() => resolve("mock async secret"), 200)
  );
}