nextjs-notion-starter-kit/lib/get-env.ts

export function getEnv(
  key: string,
  defaultValue?: string,
  env = process.env
): string {
  const value = env[key]

  if (value !== undefined) {
    return value
  }

  if (defaultValue !== undefined) {
    return defaultValue
  }

  throw new Error(`Config error: missing required env var "${key}"`)
}