Support for the experimental syntax 'classProperties' isn't currently enabled

Question

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']}]
            }
        }
    ]
}

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

replace your .babelrc file with above code. it fixed the issue for me.

In my work environment root, .babelrc file was not there. However, following entry in package.json solved the issue.

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

Note: Don't forget to exit the console and reopen before executing the npm or yarn commands.

There are two ways to work with react state:

Option 1: Just add to package.json:

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

Option 2:

1. Creta a file called .babelrc in the root folder.

Write in .babelrc:

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

2. Run:

npm i @babel/plugin-proposal-class-properties

3. Run:

npm run dev
or
npm run watch

After almost 3 hours of searching and spending time on the same error, I found that I'm using name import for React:

import { React } from 'react';

which is totally wrong. Just by switching it to:

import React from 'react';

all the error are gone. I hope this helps someone. This is my .babelrc:

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

the webpack.config.js

const path = require('path');
const devMode = process.env.Node_ENV !== 'production';
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
module.exports = {
  entry: './src/App.js',
  devtool: 'source-map',
  output: {
    path: path.resolve(__dirname, 'public'),
    filename: 'App.js'
  },
  mode: 'development',
  devServer: {
    contentBase: path.resolve(__dirname, 'public'),
    port:9090,
    open: 'google chrome',
    historyApiFallback: true
  },
  module: {
    rules: [
      {
        test: /\.m?js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader'
        }
      },{
        test: /\.(sa|sc|c)ss$/,
        use: [
          devMode ? 'style-loader' : MiniCssExtractPlugin.loader,
          {
            loader: 'css-loader',
            options: {
              modules: true,
              localIdentName: '[local]--[hash:base64:5]',
              sourceMap: true
            }
          },{
            loader: 'sass-loader'
          }
        ]
      }
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({
      filename: devMode ? '[name].css' : '[name].[hash].css',
      chunkFilename: devMode ? '[id].css' : '[id].[hash].css'
    })
  ]
}

the package.json

{
  "name": "expense-app",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "webpack",
    "serve": "webpack-dev-server"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "@babel/cli": "^7.1.2",
    "@babel/core": "^7.1.2",
    "@babel/plugin-proposal-class-properties": "^7.1.0",
    "@babel/preset-env": "^7.1.0",
    "@babel/preset-react": "^7.0.0",
    "babel-loader": "^8.0.4",
    "css-loader": "^1.0.0",
    "mini-css-extract-plugin": "^0.4.3",
    "node-sass": "^4.9.3",
    "react-router-dom": "^4.3.1",
    "sass-loader": "^7.1.0",
    "style-loader": "^0.23.1",
    "webpack": "^4.20.2",
    "webpack-cli": "^3.1.2",
    "webpack-dev-server": "^3.1.9"
  },
  "dependencies": {
    "normalize.css": "^8.0.0",
    "react": "^16.5.2",
    "react-dom": "^16.5.2"
  }
}