将这个添加到Babel config的插件将解决这个问题：

["@babel/plugin-transform-modules-commonjs"]

我有什么似乎是一个非常类似的错误，在我的Webpack 4设置（与巴别塔7）. Webpack是建立包正确，但当我试图加载文件，我得到：

Uncaught ReferenceError: exports is not defined
at eval (util.js:22)

......其中util.js是我自己的一个库。我在谷歌上搜索了很多建议用modules: false和/或sourceType: unambiguous配置babel-loader和/或将libraryTarget（在output中）设置为'umd'的解决方案，所以我尝试了如下所示的方法：

{test: /\.js$/, use:
     {
         loader: 'babel-loader',
         options:
         {
             presets: [
                 ['@babel/preset-env', {loose: true, modules: false}],
                 '@babel/react'
             ],
             sourceType: 'unambiguous'
         }
     }
}

......在我的webpack.config.js的rules部分中。上述建议的任何排列都不对我起作用。最后，它只是简单地将以下内容（从我自己的util.js文件的结尾）从：

exports.foo = foo;
exports.boo = boo;

...到：

export {foo, boo};

我将我的（工作）webpack.config.js和package.json

网页包配置. js

'use strict';
const path = require('path');

const APPDIR = 'app/';

const HtmlWebpackPlugin = require('html-webpack-plugin');
const HTMLWebpackPluginConfig = new HtmlWebpackPlugin({
    template: path.resolve(__dirname, APPDIR, 'index.html'),
    filename: 'index.html',
    inject: 'body'
});

const config = {
    context: path.resolve(__dirname, APPDIR),
    entry: './main.js',
    output: {
        path: path.resolve(__dirname, 'build'),
        filename: 'bundle.js'
    },
    module: {
        rules: [
            {test: /\.js$/, use:
             {
                 loader: 'babel-loader',
                 options:
                 {
                     presets: [
                         ['@babel/preset-env'], '@babel/react'
                     ]
                 }
             }
            },{
                test: /\.css$/,
                use: [
                    // style-loader
                    {loader: 'style-loader'},
                    // css-loader
                    {loader: 'css-loader',
                     options: {
                         modules: true
                     }
                    }]
            },{
                test: /\.(png|jpg|jpeg|gif|woff)$/,
                use: [
                    {
                        loader: 'url-loader',
                        options: {
                            limit: 8192
                        }
                    }]
            },{
                test: /\.README$/,
                use: 'null-loader'
            }
        ]
    },
    plugins: [HTMLWebpackPluginConfig],

    node: {
        fs: "empty" // This is to account for what appears to be a bug: https://github.com/josephsavona/valuable/issues/9`
    }
};

module.exports = config;

包. json

{
"name": "cubehelix-demo",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
    "clean": "rm -fr build/index.html && rm -fr build/bundle.js && npm run clean-logs",
    "build": "webpack --progress --colors",
    "build-min": "webpack --config=webpack.min.config.js",
    "dev": "webpack-dev-server --devtool eval --progress --colors --content-base build --port 8090",
    "test-build": "mkdir -p lib && babel app --out-dir lib --source-maps",
    "test": "npm run test-build && mocha --require source-map-support/register --compilers js:babel-register"
},
"keywords": [],
"author": "",
"license": "ISC",
"devDependencies": {
    "@babel/cli": "~7.6.0",
    "@babel/core": "~7.6.0",
    "@babel/preset-env": "~7.6.0",
    "@babel/preset-react": "^7.0.0",
    "@babel/register": "~7.6.0",
    "@babel/runtime": "~7.6.0",
    "babel-loader": "^8.0.6",
    "chai": "~4.2.0",
    "css-loader": "^0.23.1",
    "file-loader": "^0.9.0",
    "html-webpack-plugin": "~3.2.0",
    "mocha": "~6.2.0",
    "null-loader": "^0.1.1",
    "style-loader": "^0.13.0",
    "url-loader": "^0.5.7",
    "webpack": "~4.39.3",
    "webpack-cli": "^3.3.8",
    "webpack-dev-server": "~3.8.0"
},
"dependencies": {
    "chai": "~4.2.0",
    "classnames": "^2.2.5",
    "jquery": "~3.4.1",
    "lodash": "~4.17.15",
    "prop-types": "^15.7.2",
    "react": "~16.9.0",
    "react-custom-validators": "*",
    "react-dom": "~16.9.0",
    "react-timer-mixin": "~0.13.4"
}
}

下面是一个可能的解决方案（它对我的“导出未定义”错误有效）：https://github.com/webpack/webpack/issues/3974#issuecomment-369260590
对我来说，一个解决方案是改变babel-loader的配置，在插件下，['transform-runtime'，{“polyfill”：错误}]

不确定这是否会帮助任何人，因为我们有一个非常“特殊”的设置，我们击中了这个问题。
我们所看到的是，带有dist目录的sym链接共享库（通过package. json/ npm中的file：/ reference有效链接）被webpack错误地传递。
我们有“符号链接：false”选项，我们的解决方案是将其添加到相关module.rules[]部分中的ignore部分。
我相信这是因为，即使文件在node_modules中，并且存在符号链接，解析的路径使忽略模式没有找到它，它们再次被传递（错误）

diff --git a/project/webpack.config.js b/project/webpack.config.js
index f688472d72..a680c205e7 100644
--- a/project/webpack.config.js
+++ b/project/webpack.config.js
@@ -265,7 +265,10 @@ function webpackConfig(_target) {
        rules: [
          {
            test: /\.jsx?$/,
-            exclude: /node_modules/,
+            exclude: [
+              /node_modules/,
+              /node-lib/
+            ],
            use: {
              loader: 'babel-loader',
              options: {

（我们的共享库显示在project的兄弟目录node-lib中）

我也遇到了这个错误。对我来说，问题是我不小心在我的main.ts之外有一个main.js，导致webpack使用那个代替，并且由于某种原因没有发出很多导入的模块。删除main.js解决了这个问题。

以上Webpack配置变更均无法修复，仅源代码变更可以fix: react-native-web with react-app-rewired and react-scripts@5 and webpack@5 cause Uncaught Error: ES Modules may not assign module.exports or exports.*, Use ESM export syntax, instead: ./node_modules/@flyskywhy/react-native-gcanvas/index.js，请参阅https://github.com/webpack/webpack/issues/4039#issuecomment-419284940