vue-cli/packages/@vue/cli-service/lib/util/loadEnv.js

const fs = require('fs')

module.exports = function loadEnv (path = '.env') {
  const config = parse(fs.readFileSync(path, 'utf-8'))
  Object.keys(config).forEach(key => {
    if (typeof process.env[key] === 'undefined') {
      process.env[key] = config[key]
    }
  })
  return config
}

function parse (src) {
  const res = {}
  src.split('\n').forEach(line => {
    // matching "KEY' and 'VAL' in 'KEY=VAL'
    const keyValueArr = line.match(/^\s*([\w\.\-]+)\s*=\s*(.*)?\s*$/)
    // matched?
    if (keyValueArr != null) {
      const key = keyValueArr[1]
      let value = keyValueArr[2] || ''

      // expand newlines in quoted values
      const len = value ? value.length : 0
      if (len > 0 && value.charAt(0) === '"' && value.charAt(len - 1) === '"') {
        value = value.replace(/\\n/gm, '\n')
      }

      // remove any surrounding quotes and extra spaces
      value = value.replace(/(^['"]|['"]$)/g, '').trim()

      res[key] = value
    }
  })
  return res
}