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)
  }

  setMode (mode = 'development') {
    // by default, NODE_ENV === mode, but this can be overwritten in .env files
    process.env.NODE_ENV = mode
    // 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)
  }
}