webpack4 在webpack-config.js 中 mode設置沒有用

//webpack-config.js
const path = require('path');
module.exports = {
  mode: 'none',
  entry: './src/index.js',
  output: {
    filename: 'bundle.js',
    path: path.resolve(__dirname, '../dist')
  }　　　　　　
};    

在package.json 中scripts 增加

"build": "webpack --mode production ./src/index.js --output ./dist/bundle.js"

"dev": "webpack --mode development ./src/index.js --output ./dist/bundle.js"