Q:

Laravel mix always undefined in mix v4+

Any call to npm run after upgrading to Mix v4.x results in mix being undefined. I see errors like this on every run:
> npm run development

> @ development /project
> cross-env NODE_ENV=development node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js

/project/node_modules/webpack-cli/bin/cli.js:235
                throw err;
                ^

TypeError: Cannot read property 'js' of undefined
    at Object.<anonymous> (/project/webpack.mix.js:15:5)
    at Module._compile (/project/node_modules/v8-compile-cache/v8-compile-cache.js:178:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:732:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)
    at Module.require (internal/modules/cjs/loader.js:657:17)
    at require (/project/node_modules/v8-compile-cache/v8-compile-cache.js:159:20)
    at Object.<anonymous> (/project/node_modules/laravel-mix/setup/webpack.config.js:12:1)
    at Module._compile (/project/node_modules/v8-compile-cache/v8-compile-cache.js:178:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:732:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)
    at Module.require (internal/modules/cjs/loader.js:657:17)
    at require (/project/node_modules/v8-compile-cache/v8-compile-cache.js:159:20)
    at WEBPACK_OPTIONS (/project/node_modules/webpack-cli/bin/convert-argv.js:113:13)
    at requireConfig (/project/node_modules/webpack-cli/bin/convert-argv.js:115:6)
    at /project/node_modules/webpack-cli/bin/convert-argv.js:122:17
    at Array.forEach (<anonymous>)
    at module.exports (/project/node_modules/webpack-cli/bin/convert-argv.js:120:15)
    at yargs.parse (/project/node_modules/webpack-cli/bin/cli.js:232:39)
    at Object.parse (/project/node_modules/yargs/yargs.js:567:18)
    at /project/node_modules/webpack-cli/bin/cli.js:210:8
    at Object.<anonymous> (/project/node_modules/webpack-cli/bin/cli.js:500:3)
    at Module._compile (internal/modules/cjs/loader.js:721:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:732:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:560:12)
    at Function.Module._load (internal/modules/cjs/loader.js:552:3)
    at Module.require (internal/modules/cjs/loader.js:657:17)
    at require (internal/modules/cjs/helpers.js:22:18)
    at Object.<anonymous> (/project/node_modules/webpack/bin/webpack.js:155:2)
    at Module._compile (internal/modules/cjs/loader.js:721:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:732:10)
    at Module.load (internal/modules/cjs/loader.js:620:32)
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @ development: `cross-env NODE_ENV=development node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the @ development script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

I'm running this with Laravel 5.5. I upgraded package.json to match the version in the laravel repo that is used with mix v4.
Here is my package.json:
{
  "private": true,
  "scripts": {
    "dev": "npm run development",
    "development": "cross-env NODE_ENV=development node_modules/webpack/bin/webpack.js --progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js",
    "watch": "npm run development -- --watch",
    "watch-poll": "npm run watch -- --watch-poll",
    "hot": "cross-env NODE_ENV=development node_modules/webpack-dev-server/bin/webpack-dev-server.js --inline --hot --config=node_modules/laravel-mix/setup/webpack.config.js",
    "prod": "npm run production",
    "production": "cross-env NODE_ENV=production node_modules/webpack/bin/webpack.js --no-progress --hide-modules --config=node_modules/laravel-mix/setup/webpack.config.js"
  },
  "devDependencies": {
    "accounting": "^0.4.1",
    "axios": "^0.18.0",
    "bootstrap": "^4.1.3",
    "bootstrap-sass": "^3.4.0",
    "cross-env": "^5.2.0",
    "fancybox": "^3.0.1",
    "jquery": "^3.3.1",
    "jquery-slimscroll": "^1.3.8",
    "ladda": "^2.0.1",
    "laravel-mix": "^4.0.13",
    "locutus": "^2.0.10",
    "metismenu": "^2.7.9",
    "moment": "^2.23.0",
    "vue": "^2.5.21",
    "webpack": "^4.28.3",
    "webpack-cli": "^3.2.0"
  },
  "dependencies": {
    "ajv": "^6.6.2",
    "bootstrap-vue": "^2.0.0-rc.11",
    "extract-text-webpack-plugin": "^4.0.0-beta.0",
    "imagemin": "^6.0.0",
    "jsonexport": "^2.4.1",
    "laravel-echo": "^1.5.2",
    "lodash": "^4.17.11",
    "node-sass": "^4.11.0",
    "pusher-js": "^4.3.1",
    "sweetalert2": "^7.32.4",
    "vue-api-query": "^1.2.0",
    "vue-events": "^3.1.0",
    "vue-multiselect": "^2.1.3",
    "vue-sweetalert2": "^1.6.3",
    "vuetable-2": "^2.0.0-beta.4",
    "vuex": "^3.0.1"
  }
}

My webpack.mix.js:
const { mix } = require('laravel-mix');

mix.js('resources/assets/js/app.js', 'public/js')
    .sass('resources/assets/sass/app.scss', 'public/css')
    .version()
    .sourceMaps();

I've tried deleting node_modules and yarn install again but that doesn't fix it.
If I downgrade to v3, there are no issues other than problems compiling some SASS.

npm v6.4.1
node v11.5.0
yarn v1.12.3

A:

Without seeing your webpack.mix.js file it's hard to say what the problem could be. I cloned the Laravel 5.5 branch and upgraded to version ^4.0.13 of laravel-mix and successfully ran both yarn dev and yarn prod, although I am using more recent versions of npm and node.

NPM 6.5.0-next.0
Node 11.6.0
Yarn 1.12.3

Here's a GitHub repo to demo.

Update
// TypeError: Cannot read property 'js' of undefined
const { mix } = require('laravel-mix');

// works
const mix = require('laravel-mix');

