Change

"plugins": [    "@babel/plugin-proposal-class-properties"  ]

To

"plugins": [    [      "@babel/plugin-proposal-class-properties",      {        "loose": true      }    ]  ]

This worked for me

First install the: @babel/plugin-proposal-class-properties as dev dependency:

npm install @babel/plugin-proposal-class-properties --save-dev

Then edit your .babelrc so it will be exact like this:

{  "presets": [      "@babel/preset-env",      "@babel/preset-react"  ],  "plugins": [      [        "@babel/plugin-proposal-class-properties"      ]  ]}

.babelrc file located in the root directory, where package.json is.

Note that you should re-start your webpack dev server to changes take affect.

Solution for webpack project

I just solve this problem by adding @babel/plugin-proposal-class-properties into webpack config plugin.The module section of my webpack.config.js looks like this

module: {    rules: [        {            test: path.join(__dirname, '.'),            exclude: /(node_modules)/,            loader: 'babel-loader',            options: {                presets: ['@babel/preset-env',                          '@babel/react',{                          'plugins': ['@babel/plugin-proposal-class-properties']}]            }        }    ]}