什么是热更新？webpack中配置服务热更新的实现

const path = require('path')    //path是一个常量不能更改  ，path 需要引入var webpack = require('webpack')
module.exports = {  // bundle入口
  entry:{
    entry:'./src/entry.js',    //下面的entry是随便起的名字
    entry2:'./src/entry2.js'    //有两个入口也要有两个出口
  },  // bundle输出
  output: {
    path: path.resolve(__dirname, 'dist'),    //绝对路径
    filename: '[name].js' //可重命名        当有多个入口文件时，出口文件用name，说明打包的出口文件和入口文件名相同
  },
  module:{},
  plugins:[],
  devServer:{
    contentBase:path.resolve(__dirname,'dist'),
    host:'10.212.109.18',
    compress:true,
    port:8087
  }
}

> y@1.0.0 server F:\webLearn\webpackLearn
> webpack-dev-server

The CLI moved into a separate package: webpack-cli.
Please install 'webpack-cli' in addition to webpack itself to use the CLI.-> When using npm: npm install webpack-cli -D
-> When using yarn: yarn add webpack-cli -Dmodule.js:549
    throw err;
    ^Error: Cannot find module 'webpack-cli/bin/config-yargs'
    at Function.Module._resolveFilename (module.js:547:15)
    at Function.Module._load (module.js:474:25)
    at Module.require (module.js:596:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (F:\webLearn\webpackLearn\node_modules\webpack-dev-server\bin\webpack-dev-server.js:65:1)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)
    at Function.Module._load (module.js:497:3)
npm ERR! code ELIFECYCLE
npm ERR! errno 1npm ERR! y@1.0.0 server: `webpack-dev-server`
npm ERR! Exit status 1npm ERR!
npm ERR! Failed at the y@1.0.0 server script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     C:\Users\文件夹名称\AppData\Roaming\npm-cache\_logs\2018-07-10T08_59_23_339Z-debug.log

> y@1.0.0 server F:\webLearn\webpackLearn
> webpack-dev-server

i ｢wds｣: Project is running at http://10.212.109.18:8087/i ｢wds｣: webpack output is served from /
i ｢wds｣: Content not from webpack is served from F:\webLearn\webpackLearn\dist
‼ ｢wdm｣: Hash: 0a1133d150c765ff1b91
Version: webpack 4.15.1Time: 12622ms
Built at: 2018-07-10 17:01:51
    Asset     Size  Chunks             Chunk Names
entry2.js  139 KiB       0  [emitted]  entry2
 entry.js  139 KiB       1  [emitted]  entry
Entrypoint entry = entry.js
Entrypoint entry2 = entry2.js
 [3] (webpack)/hot/emitter.js 77 bytes {0} {1} [built]
 [4] (webpack)/hot/log.js 1010 bytes {0} {1} [optional] [built]
 [5] (webpack)/hot sync nonrecursive ^\.\/log$ 170 bytes {0} {1} [built]
 [8] ./node_modules/html-entities/index.js 231 bytes {0} {1} [built]
[10] (webpack)-dev-server/client/overlay.js 3.58 KiB {0} {1} [built]
[12] (webpack)-dev-server/client/socket.js 1.05 KiB {0} {1} [built]
[13] ./node_modules/loglevel/lib/loglevel.js 7.68 KiB {0} {1} [built]
[14] ./node_modules/ansi-regex/index.js 135 bytes {0} {1} [built]
[15] ./node_modules/strip-ansi/index.js 161 bytes {0} {1} [built]
[22] ./node_modules/url/url.js 22.8 KiB {0} {1} [built]
[23] (webpack)-dev-server/client?http://10.212.109.18:8087 7.75 KiB {0} {1} [built][24] ./src/entry2.js 23 bytes {0} [built]
[25] multi (webpack)-dev-server/client?http://10.212.109.18:8087 ./src/entry2.js 40 bytes {0} [built][26] ./src/entry.js 60 bytes {1} [built]
[27] multi (webpack)-dev-server/client?http://10.212.109.18:8087 ./src/entry.js 40 bytes {1} [built]
    + 13 hidden modules

WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for this value. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/concepts/mode/i ｢wdm｣: Compiled with warnings.