`TypeError: Cannot read properties of undefined (reading 'then')` when using ESM to export config

[palapapa]

Bug report

What is the current behavior?
Running npx webpack --config config/webpack.config.js gives this error:

[webpack-cli] TypeError: Cannot read properties of undefined (reading 'then')
    at WebpackCLI.isPromise (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:27:29)
    at loadConfigByPath (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:1354:26)
    at async Promise.all (index 0)
    at async WebpackCLI.loadConfig (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:1375:35)
    at async WebpackCLI.createCompiler (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:1643:22)
    at async WebpackCLI.runWebpack (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:1738:20)
    at async Command.<anonymous> (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:885:21)
    at async Command.parseAsync (C:\JavaScript\wiki-translator\node_modules\webpack-cli\node_modules\commander\lib\command.js:935:5)
    at async Command.<anonymous> (C:\JavaScript\wiki-translator\node_modules\webpack-cli\lib\webpack-cli.js:1292:13)
    at async Command.parseAsync (C:\JavaScript\wiki-translator\node_modules\webpack-cli\node_modules\commander\lib\command.js:935:5)

webpack.config.js:

export let config =
{
    "entry": "dist/popup.js"
}

dist/popup.js:

"use strict";
console.log("Hello World!");

package.json:

{
    "name": "wiki-translator",
    "version": "0.1.0",
    "description": "A Chrome extension that automatically detects the longest language of a Wikipedia atricle and translates it.",
    "private": true,
    "scripts":
    {
        "watch": "tsc-watch --project config/tsconfig.json --onSuccess \"webpack --mode=development --config config/webpack.config.js\"",
        "build": "tsc --project config/tsconfig.json; webpack --mode=production --config config/webpack.config.js"
    },
    "devDependencies":
    {
        "@types/chrome": "^0.0.181",
        "copy-webpack-plugin": "^10.2.4",
        "css-loader": "^6.7.3",
        "file-loader": "^6.2.0",
        "mini-css-extract-plugin": "^2.7.5",
        "prettier": "^2.8.4",
        "ts-loader": "^9.2.8",
        "tsc-watch": "^6.0.0",
        "typescript": "^4.6.3",
        "webpack": "^5.76.2",
        "webpack-cli": "^5.0.1",
        "webpack-merge": "^5.8.0"
    },
    "type": "module"
}

If the current behavior is a bug, please provide the steps to reproduce.
Run npx webpack --config config/webpack.config.js in this repo:
wiki-translator.zip

What is the expected behavior?
Webpack should run successfully.

Other relevant information:
webpack version: 5.76.2
webpack-cli version: 5.0.1
Node.js version: 18.15.0
Operating System: Windows 10 10.0.19044

[alexander-akait]

We support only default export, please use:

export default
{
    "entry": "dist/popup.js"
}

[alexander-akait]

We don't know what is config named export so we don't support named exports