serverless/lib/cli/load-dotenv.js

import dotenv from 'dotenv'
import dotenvExpand from 'dotenv-expand'
import path from 'path'
import ServerlessError from '../serverless-error.js'

const isMissingFileError = (error) => error.code === 'ENOENT'

const throwDotenvError = (error, filePath) => {
  const errorMessage = `Failed to load environment variables from "${filePath}": ${error}`
  throw new ServerlessError(errorMessage, 'DOTENV_LOAD_ERROR')
}

const loadDotenvFrom = (envFilePath) => {
  const config = dotenv.config({ path: envFilePath })
  return dotenvExpand.expand(config)
}

export default (stage) => {
  const defaultEnvFilePath = path.join(process.cwd(), '.env')
  const stageEnvFilePath = path.join(process.cwd(), `.env.${stage}`)

  const { error: stageEnvResultError } = loadDotenvFrom(stageEnvFilePath)

  if (!stageEnvResultError) return

  if (!isMissingFileError(stageEnvResultError)) {
    throwDotenvError(stageEnvResultError, stageEnvFilePath)
  }

  const { error: defaultEnvResultError } = loadDotenvFrom(defaultEnvFilePath)

  if (defaultEnvResultError && !isMissingFileError(defaultEnvResultError)) {
    throwDotenvError(defaultEnvResultError, defaultEnvFilePath)
  }
}