Ошибка сборки Webpack для начальной загрузки

Когда я приступил к сборке, появляется следующее «Предупреждение». Как я могу это исправить?

И нет ли другой команды для сборки в реальном времени, кроме "$ npm run build"? пример) $ sass - смотреть

Или предварительный просмотр в реальном времени

  $ npm run build

    > [email protected] build /Users/suk/myweb
    > webpack

    Hash: 62314a5e1bd8f234994f
    Version: webpack 4.16.1
    Time: 7100ms
    Built at: 2018-07-18 19:27:20
        Asset     Size  Chunks                    Chunk Names
    bundle.js  328 KiB       0  [emitted]  [big]  main
    [0] ./src/app.js 45 bytes {0} [built]
    [4] (webpack)/buildin/global.js 489 bytes {0} [built]
    [5] ./src/scss/app.scss 1.35 KiB {0} [built]
    [6] ./node_modules/css-loader!./node_modules/postcss-loader/lib??ref--4-2!./node_modules/sass-loader/lib/loader.js!./src/scss/app.scss 165 KiB {0} [built]
        + 6 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/

    WARNING in asset size limit: The following asset(s) exceed the recommended size limit (244 KiB).
    This can impact web performance.
    Assets: 
      bundle.js (328 KiB)

    WARNING in entrypoint size limit: The following entrypoint(s) combined asset size exceeds the recommended limit (244 KiB). This can impact web performance.
    Entrypoints:
      main (328 KiB)
          bundle.js


    WARNING in webpack performance recommendations: 
    You can limit the size of your bundles by using import() or require.ensure to lazy load some parts of your application.
    For more info visit https://webpack.js.org/guides/code-splitting/