这个问题可能在其他地方得到了回答，但在标记为重复之前，请帮助我。我指的是使用 react 和 d3 的以下代码笔：https ://codepen.io/swizec/pen/oYNvpQ

但是，我无法弄清楚这个代码笔如何处理在没有任何关键字的情况下在类中声明的变量：

class Colors extends Component {
 colors = d3.schemeCategory20;
  width = d3.scaleBand()
            .domain(d3.range(20));

   ....
}

当我尝试执行此代码时，出现以下错误：

./app/components/D3IndentedTree/Chloreophath.js
Module build failed: SyntaxError: Unexpected token (13:11)

  11 | // Draws an entire color scale
  12 | class Colors extends Component {
> 13 |     colors = d3.schemeCategory20;
     |            ^
  14 |     width = d3.scaleBand()
  15 |         .domain(d3.range(20));
  16 | 

我无法弄清楚为什么会出现此错误。我知道您不能直接在类中声明类的变量/属性。但是代码笔是怎么工作的呢？

提前致谢！

更新：添加 webpack.config.js 文件：

    var path = require('path');
var webpack = require('webpack');
var HtmlWebpackPlugin = require('html-webpack-plugin');

module.exports = {
    entry: './app/index.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'index_bundle.js',
        publicPath: '/'
    },
    module: {
        rules: [
            { test: /\.(js)$/, use: 'babel-loader' },
            { test: /\.css$/, use: [ 'style-loader', 'css-loader'] },
            {
                test: /\.png$/,
               loader: "url-loader?limit=100000"
             },
             {
               test: /\.jpg$/,
               loader: "file-loader"
             },
             {
              test: /\.(woff|woff2)(\?v=\d+\.\d+\.\d+)?$/,
               loader: 'url-loader? limit=10000&mimetype=application/font-woff'
              },
              {
               test: /\.ttf(\?v=\d+\.\d+\.\d+)?$/,
               loader: 'url-loader?limit=10000&mimetype=application/octet-stream'
              },
              {
               test: /\.eot(\?v=\d+\.\d+\.\d+)?$/,
               loader: 'file-loader'
               },
               {
               test: /\.svg(\?v=\d+\.\d+\.\d+)?$/,
               loader: 'url-loader?limit=10000&mimetype=image/svg+xml'
              }
        ]
    },
    plugins: [new HtmlWebpackPlugin({
        template: 'app/index.html'
    }),
    new webpack.ProvidePlugin({
        "React": "react",
      }),],
    devServer: {
        historyApiFallback: true
    }
};