vue-cli/packages/@vue/cli-service/lib/PluginAPI.js

const path = require('path')

module.exports = class PluginAPI {
  constructor (id, service) {
    this.id = id
    this.service = service
  }

  resolve (_path) {
    return path.resolve(this.service.context, _path)
  }

  hasPlugin (id) {
    return this.service.plugins.some(p => p.id === id)
  }

  // set project mode.
  // this should be called by any registered command as early as possible.
  setMode (mode) {
    process.env.VUE_CLI_MODE = mode
    // by default, NODE_ENV and BABEL_ENV are set to "development" unless mode
    // is production or test. However this can be overwritten in .env files.
    process.env.NODE_ENV = process.env.BABE_ENV =
      (mode === 'production' || mode === 'test')
        ? mode
        : 'development'
    // load .env files based on mode
    this.service.loadEnv(mode)
  }

  registerCommand (name, opts, fn) {
    if (typeof opts === 'function') {
      fn = opts
      opts = null
    }
    this.service.commands[name] = { fn, opts }
  }

  chainWebpack (fn) {
    this.service.webpackChainFns.push(fn)
  }

  configureWebpack (fn) {
    this.service.webpackRawConfigFns.push(fn)
  }

  resolveWebpackConfig () {
    return this.service.resolveWebpackConfig()
  }

  configureDevServer (fn) {
    this.service.devServerConfigFns.push(fn)
  }
}