Develop Reference

Webpack 5 React component library UMD bundle with SourceMaps

I am attempting to make a webpack 5 build process to create a react component library I just had a couple of things cannot seem to get working.
#1) The webpack build command works fine, and when using the inline-source-map option I can
SEE the data URL embeded in the outputted build file but when I ever I attempt to publish and test this library on NPM I always get obfuscated errors without original lines of code so I can't even tell where the errors are; what else am I missing to activate source-maps? I am using Chrome dev tools and it doesn't even tell me a source map is available for that code...
#2) Another issue I am having is after building this with webpack into the dist folder; I start another CRA test app and try to pull components out of the built library but all I get are these errors.
./src/dist/index.js
Line 1:1: Expected an assignment or function call and instead saw an expression no-unused-expressions
Line 1:112: 'define' is not defined no-undef
Line 1:123: 'define' is not defined no-undef
Line 1:190: Unexpected use of 'self' no-restricted-globals
Line 1:466: Expected an assignment or function call and instead saw an expression no-unused-expressions
Line 1:631: Expected an assignment or function call and instead saw an expression no-unused-expressions
I am aware webpack 5 stopped bundling polyfills for Node but shouldn't this code run
if I place it in the src directory of a CRA application? This is bundled code shouldn't it work in the browser/ in another React application? I targeted UMD so I thought it would work in this environment
here is all the necessary info
Webpack.config.js
const path = require("path");
const { CleanWebpackPlugin } = require("clean-webpack-plugin");
const nodeExternals = require("webpack-node-externals");
module.exports = {
entry: "./src/index.js",
devtool: "inline-source-map",
externals: [nodeExternals()],
output: {
filename: "index.js",
path: path.resolve(__dirname, "dist"),
library: {
name: "test",
type: "umd",
},
},
plugins: [new CleanWebpackPlugin()],
module: {
rules: [
{
test: /\.(js|jsx)$/,
exclude: /node_modules/,
use: {
loader: "babel-loader",
options: {
presets: ["#babel/preset-env", "#babel/preset-react"],
},
},
},
{
test: /\.scss$/,
use: ["style-loader", "css-loader", "sass-loader"],
include: path.resolve(__dirname, "./src"),
},
],
},
};
Package.json
{
"name": "test-lib",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"start": "start-storybook",
"build": "webpack --mode production"
},
"keywords": [],
"author": "",
"license": "ISC",
"dependencies": {
"prop-types": "^15.7.2",
"react": "^17.0.2",
"react-dom": "^17.0.2"
},
"devDependencies": {
"#babel/core": "^7.15.8",
"#babel/preset-env": "^7.15.8",
"#babel/preset-react": "^7.14.5",
"#storybook/addon-knobs": "^6.3.1",
"#storybook/react": "^6.3.10",
"babel-loader": "^8.2.2",
"clean-webpack-plugin": "^4.0.0",
"node-sass": "^6.0.1",
"path": "^0.12.7",
"sass-loader": "^12.1.0",
"webpack": "^5.58.0",
"webpack-cli": "^4.9.0",
"webpack-node-externals": "^3.0.0"
}
}
Button.js (sample component)
import React from 'react'
import PropTypes from 'prop-types';
const Button = ({message = 'Hello world'}) => (
<button>{message}</button>
)
Button.propTypes = {
message: PropTypes.string.isRequired
}
export default Button
Build entry point (index.js)
export { default as Button } from "./components/Button";

Webpack with babel and handlebars files

REVISED
So I am fairly new to this and I am struggling for some time to resolve this, so I would really appreciate help.
Goal: Create dynamic web-page that is using handlebars and D3 for dynamic text and visuals.
What I achieved until now: Use json file stored within to do some data manipulation and render data with hbs and express. Created simple bar chart that uses data from the previous file.
Issue: I am not sure how to completely set up webpack so I can actually see how my page looks like. If I just add script with D3 visuals to hbs I am getting require is not defined, which I get since it's not supported on client side.
folder structure
|main
|data
|data.json
|src
|index.js
|visuals.js
|templates
|views
|index.hbs
|node_modules
|package.json
|package-lock.json
|webpack.config.js
|babel.config.json
My code until now (there might be to many things here because I tried a lot of things plus I was anonymizing as I have sensitive items)
index.js:
const express = require("express");
const fs = require("fs");
const path = require("path");
const hbs = require("hbs");
const Data = JSON.parse(
fs.readFileSync(path.resolve(__dirname, "../data/data.json")).toString()
);
//Some data manipulation
module.exports = clusters; //array to be used in the other file
const app = express();
const publicDirectoryPath = path.join(__dirname, "..");
const viewPath = path.join(__dirname, "../templates/views");
app.set("view engine", "hbs");
app.set("views", viewPath);
app.use(express.static(publicDirectoryPath));
app.get("", (req, res) => {
res.render("index", {
data1: data1,
data2:data2,
});
});
Beginning of visuals.js
const d3 = require("d3");
var dt = require("./index.js");
const clusters = dt.clusters;
webpack.config.js
const path = require("path");
const HandlebarsPlugin = require("handlebars-webpack-plugin");
module.exports = {
entry: [
path.resolve(__dirname, "./src/visuals.js"),
path.resolve(__dirname, "./src/index.js"),
],
output: {
path: path.resolve(__dirname, "./public"),
filename: "bundle.js",
},
module: {
rules: [
{ test: /\.handlebars$/, loader: "handlebars-loader" },
{
test: /\.js$/,
exclude: /node_modules/,
use: ["babel-loader"],
},
],
},
resolve: {//I had errors and warnings with modules, below solved it
modules: [path.resolve(__dirname), "node_modules/"],
extensions: [".js", ".json"],
descriptionFiles: ["package.json"],
},
fallback: {
stream: false,
http: false,
buffer: false,
crypto: false,
zlib: false,
fs: false,
net: "empty",
},
},
plugins: [
new HandlebarsPlugin({//can't say I understood from documentation the setup for hbs
entry: path.resolve(__dirname, "./templates/views/index.hbs"),
output: path.resolve(__dirname, "./public/index.html"),
data: path.resolve(__dirname, "./data/data.json"),
onBeforeSetup: function (Handlebars) {},
onBeforeCompile: function (Handlebars, templateContent) {},
onBeforeRender: function (Handlebars, data, filename) {},
onBeforeSave: function (Handlebars, resultHtml, filename) {},
onDone: function (Handlebars, filename) {},
}),
],
};
package.json
{
"name": "triana_plus",
"version": "1.0.0",
"description": "",
"main": "index.js",
"scripts": {
"dev": "webpack --mode development",
"start": "webpack serve --config ./webpack.config.js --open chrome"
},
"author": "",
"license": "ISC",
"devDependencies": {
"#babel/core": "^7.12.10",
"#babel/preset-env": "^7.12.10",
"babel-loader": "^8.2.2",
"handlebars-webpack-plugin": "^2.2.1",
"webpack": "^5.10.1",
"webpack-cli": "^4.2.0",
"webpack-dev-server": "^3.11.0",
"webpack-node-externals": "^2.5.2"
},
"dependencies": {
"d3": "^6.3.1",
"express": "^4.17.1",
"fs": "0.0.1-security",
"handlebars-loader": "^1.7.1",
"hbs": "^4.1.1",
"net": "^1.0.2",
"path": "^0.12.7",
"request": "^2.88.2"
}
}
babel.config.json
{
"presets": ["#babel/preset-env"]
}

What the error you posted means is that webpack is being asked to create some JS bundles that reference node packages like http. Referencing packages like that in client-side code bundles won't work since it cannot package node modules in the JS artifacts.
Make sure that no node-specific packages are required/imported for files you're bundling through webpack. Since it looks like you're creating an express app, keep the express-specific files outside of the folder visited by webpack.

Svelte 3: Could not import modules when unit testing

I'm trying to test a Svelte component with Jest. This component works fine in the browser, but unit test fails with importing modules.
For example, when running Jest, import uuid from 'uuid' compiled as const { default: uuid } = require("uuid");, and calling uuid.v4() causes TypeError: Cannot read property 'v4' of undefined. When I use import * as uuid from 'uuid' or const uuid = require('uuid'), Jest unit test passes, but it doesn't work in the browser.
How can I deal with this issue? Any information would greatly help. Thank you.
package.json:
{
"name": "svelte-app",
"version": "1.0.0",
"scripts": {
"build": "rollup -c",
"dev": "rollup -c -w",
"start": "firebase serve --only hosting"
},
"devDependencies": {
"#rollup/plugin-json": "^4.0.0",
"#testing-library/jest-dom": "^5.1.1",
"#testing-library/svelte": "^1.11.0",
"bulma": "^0.8.0",
"eslint": "^6.7.0",
"eslint-config-standard": "^14.1.0",
"eslint-plugin-import": "^2.18.2",
"eslint-plugin-jest": "^23.0.4",
"eslint-plugin-node": "^10.0.0",
"eslint-plugin-promise": "^4.2.1",
"eslint-plugin-standard": "^4.0.1",
"eslint-plugin-svelte3": "^2.7.3",
"jest-transform-svelte": "^2.1.1",
"node-sass": "^4.13.1",
"rollup": "^1.12.0",
"rollup-jest": "0.0.2",
"rollup-plugin-commonjs": "^10.0.0",
"rollup-plugin-livereload": "^1.0.0",
"rollup-plugin-node-builtins": "^2.1.2",
"rollup-plugin-node-globals": "^1.4.0",
"rollup-plugin-node-resolve": "^5.2.0",
"rollup-plugin-svelte": "^5.0.3",
"rollup-plugin-terser": "^5.1.2",
"sirv-cli": "^0.4.5",
"svelma": "^0.3.2",
"svelte": "^3.18.2",
"svelte-preprocess": "^3.4.0"
},
"dependencies": {
"firebase": "^7.8.2"
},
"private": true
}
rollup.config.js
import json from '#rollup/plugin-json'
import commonjs from 'rollup-plugin-commonjs'
import builtins from 'rollup-plugin-node-builtins'
import globals from 'rollup-plugin-node-globals'
import livereload from 'rollup-plugin-livereload'
import resolve from 'rollup-plugin-node-resolve'
import svelte from 'rollup-plugin-svelte'
import { terser } from 'rollup-plugin-terser'
import preprocess from 'svelte-preprocess'
const production = !process.env.ROLLUP_WATCH
export default {
input: 'src/main.js',
output: {
sourcemap: true,
format: 'iife',
name: 'app',
file: 'public/build/bundle.js',
},
plugins: [
// https://github.com/rollup/plugins/tree/master/packages/json
json(),
svelte({
// enable run-time checks when not in production
dev: !production,
// we'll extract any component CSS out into
// a separate file — better for performance
css: css => {
css.write('public/build/bundle.css')
},
preprocess: preprocess(),
}),
// If you have external dependencies installed from
// npm, you'll most likely need these plugins. In
// some cases you'll need additional configuration —
// consult the documentation for details:
// https://github.com/rollup/rollup-plugin-commonjs
resolve({
browser: true,
dedupe: importee => importee === 'svelte' || importee.startsWith('svelte/'),
}),
commonjs(),
globals(),
builtins(),
// In dev mode, call `npm run start` once
// the bundle has been generated
!production && serve(),
// Watch the `public` directory and refresh the
// browser on changes when not in production
!production && livereload('public'),
// If we're building for production (npm run build
// instead of npm run dev), minify
production && terser(),
],
watch: {
clearScreen: false,
},
}
function serve () {
let started = false
return {
writeBundle () {
if (!started) {
started = true
require('child_process').spawn('npm', ['run', 'start'], {
stdio: ['ignore', 'inherit', 'inherit'],
shell: true,
})
}
},
}
}
jest.config.js
const sveltePreprocess = require('svelte-preprocess')
module.exports = {
displayName: { name: 'web', color: 'magentaBright' },
moduleFileExtensions: [
'js',
'json',
'svelte',
],
preset: 'rollup-jest',
transform: {
'\\.js$': 'rollup-jest',
'\\.svelte$': ['jest-transform-svelte', { preprocess: sveltePreprocess(), debug: true }],
},
setupFilesAfterEnv: ['#testing-library/jest-dom/extend-expect'],
}

It worked for me.
The issue seems like jest unable to resolve uuid while building the code at runtime.
Which is quite obvious because by default jest ignore node_modules packages.
I faced similar issues and resolved it. The approach is by configuration inform JEST that it has to include node_modules packages as well. In my project i used rollup-plugin-babel.
This is the babel plugin configuration
...
...
babel({
extensions: [ '.js', '.mjs', '.html', '.svelte' ],
runtimeHelpers: true,
exclude: [ 'node_modules/#babel/**', 'node_modules/core-js/**' ],
presets: [
[
'#babel/preset-env',
{
targets: '> 0.25%, not dead',
useBuiltIns: 'usage',
corejs: 3
}
]
]
})
And I've added babel-jest for transforming the jest.config.js
module.exports = {
preset: 'jest-puppeteer', //ignore the preset part, I used for puppeteer
transform: {
'^.+\\.js?$': require.resolve('babel-jest'),
"^.+\\.ts?$": "ts-jest" // this part is only required if you have typescript project
}
};
DO Not forget to install this packages like babel-jest, rollup-plugin-babel before using it.

I have been facing this same issue and have a work around by mocking the module in the test file and giving it a default key.
jest.mock('uuid', () => ({
default: {
v4: jest.fn(),
},
}))
Another way that seems to work is to destructure the import in the component file.
import { v4 as uuidv4 } from 'uuid'

Self answer:
Finally, I wrote a little preprocessor to replace import foo from 'foo' -> import * as foo from 'foo'
svelteJestPreprocessor.js
const svelteJestPreprocessor = () => ({
// replace `import foo from 'foo'` -> `import * as foo from 'foo'`
script: ({ content }) => ({
// process each line of code
code: content.split('\n').map(line =>
// pass: no import, import with {}, import svelte component
(!line.match(/\s*import/)) || (line.match(/{/)) || (line.match(/\.svelte/)) ? line
: line.replace(/import/, 'import * as'),
).join('\n'),
}),
})
module.exports = svelteJestPreprocessor
jest.config.js
const svelteJestPreprocessor = require('./svelteJestPreprocessor')
const sveltePreprocess = require('svelte-preprocess')
module.exports = {
moduleFileExtensions: [
'js',
'json',
'svelte',
],
preset: 'rollup-jest',
transform: {
'\\.js$': 'rollup-jest',
'\\.svelte$': ['jest-transform-svelte', {
preprocess: [
svelteJestPreprocessor(),
sveltePreprocess(),
],
}],
},
setupFilesAfterEnv: ['#testing-library/jest-dom/extend-expect'],
}
This is an unwanted workaround, but it works for now.

Babel and Webpack are throwing "Can't resolve 'regenerator-runtime/runtime'"

I'm working on a browser-based project that needs to be IE11-compatible (sigh). Webpack is choking on async/await. Here is my console's output:
Based on your code and targets, added regenerator-runtime.
...
Module not found: Error: Can't resolve 'regenerator-runtime/runtime'
I've looked at many SO questions similar to mine, without luck. Many recommend using #babel/polyfill which I am avoiding since it has been deprecated.
What is causing this issue? I expect it could be fixed by manually importing regenerator-runtime/runtime, but it seems one of the main selling points of babel-env is NOT having to manually import polyfills, so I assume I'm missing a step. Thank you!
Here is what I am attempting to run, which is being imported into another file:
class Fetcher {
constructor() {
this.form = document.querySelector('form');
this.form.addEventListener('submit', this.onSubmit.bind(this));
}
async onSubmit(event) {
event.preventDefault();
const apiResponse = await fetch(`${WP_url}/api`);
const apiJson = await apiResponse.json();
console.log(apiJson);
}
}
export default Fetcher;
webpack.config.js:
const path = require('path');
function pathTo(filepath) {
return path.join(__dirname, filepath);
}
module.exports = function(env, argv) {
const isProd = Boolean(argv.mode === 'production');
const config = {
entry: {
index: [
pathTo('index.js'),
],
},
externals: {
jquery: 'jQuery',
},
module: {
rules: [
{
test: /\.js$/,
exclude: /node_modules/,
loader: 'babel-loader',
options: {
presets: [
[
'#babel/preset-env',
{
corejs: 3,
debug: true,
targets: {
browsers: [
'IE 11',
],
},
useBuiltIns: 'usage',
},
],
],
},
},
],
},
optimization: {
minimize: isProd,
},
output: {
filename: '[name].js',
path: pathTo('web'),
},
};
return config;
};
package.json
{
"private": true,
"dependencies": {
"core-js": "^3.4.1",
"focus-within-polyfill": "^5.0.5"
},
"devDependencies": {
"#babel/core": "^7.7.2",
"#babel/preset-env": "^7.7.1",
"babel-eslint": "^10.0.3",
"babel-loader": "^8.0.6",
"css-loader": "^3.2.0",
"eslint": "^6.6.0",
"mini-css-extract-plugin": "^0.8.0",
"node-sass": "^4.13.0",
"sass-loader": "^8.0.0",
"webpack": "^4.41.2",
"webpack-cli": "^3.3.10"
},
"scripts": {
"dev": "webpack --mode=development --display-modules",
"dev:watch": "npm run dev -- --watch",
"prod": "webpack --mode=production --display-modules",
"prod:watch": "npm run prod -- --watch"
}
}

Simply running npm i regenerator-runtime fixed it, actually.
With useBuiltIns: 'usage', having all the import statements wasn't necessary I guess.
¯\_(ツ)_/¯

Just add import 'regenerator-runtime/runtime' in the file where you have the async/await.

In my case, your answer wasn't enough and I needed to set babel sourceType to unambiguous as suggested here to allow a correct compilation of the project. This option was required because the #babel/runtime/regenerator/index.js file exports its reference using module.exports = require("regenerator-runtime"); that breaks ES6 compilation.
Another useful note resolving a similar but unrelated compilation issue was to use /node_modules\/(css-loader|#babel|core-js|promise-polyfill|webpack|html-webpack-plugin|whatwg-fetch)\// as exclude for the babel rule to avoid resolution loops without loosing external libraries compilation (when necessary) as suggested here.
TL;DR
In your specific case the babel rule would become:
{
test: /\.js$/,
exclude: /node_modules/,
loader: 'babel-loader',
options: {
sourceType: 'unambiguous',
presets: [
['babel/preset-env', {
corejs: 3,
debug: true,
targets: {
browsers: [ 'IE 11', ],
},
useBuiltIns: 'usage',
}],
],
},
},

I started getting this error when added transpileDependencies: ['openplayerjs'], to my vue.config.js, as Babel with #vue/cli-plugin-babel/preset won't touch dependencies in node_modules, go figure why. Installing regenerator-runtime did not help, and I thought the issue lied in Yarn's pnp modules:
This dependency was not found:
* regenerator-runtime/runtime.js in /Volumes/Backup/home/Documents/.yarn/unplugged/openplayerjs-npm-2.9.3-aa4692035d/node_modules/openplayerjs/dist/esm/media.js, /Volumes/Backup/home/Documents/.yarn/unplugged/openplayerjs-npm-2.9.3-aa4692035d/node_modules/openplayerjs/dist/esm/media/ads.js and 1 other
To install it, you can run: npm install --save regenerator-runtime/runtime.js
so I tried unplugging everything via yarn unplug <module_name>, still did not work.
Eventually changing useBuiltIns: 'usage' to useBuiltIns: 'entry' in babel.config.js solved it straight away.

Following the official documentation:
Install required packages.
npm install --save-dev #babel/plugin-transform-runtime
npm install --save #babel/runtime
Add plugin to the configuration file.
{
"plugins": ["#babel/plugin-transform-runtime"]
}
This is officially recommended method, more available in the mentioned documentation.

webpack imported module is not a constructor

I created a small JS module which I intend to make an npm package, but for now is just on GitHub. This module is written in ES6 and SCSS, and is thus relying on webpack and babel for transpilation.
To test it, I created a separate project with a similar setup (webpack and babel). After npm installing my module, when trying to import it into my index.js, I get the following error in Chrome Developer Tools: (with x being my module's name)
index.js:11 Uncaught TypeError: x__WEBPACK_IMPORTED_MODULE_1___default.a is not a constructor
at eval (index.js:11)
at Object../src/index.js (main.js:368)
at __webpack_require__ (main.js:20)
at eval (webpack:///multi_(:8081/webpack)-dev-server/client?:2:18)
at Object.0 (main.js:390)
at __webpack_require__ (main.js:20)
at main.js:69
at main.js:72
I've looked through countless answers and tried countless solutions, to no avail. My module's setup is as follows.
.babelrc
{
"presets": [
["env", {
"targets": {
"browsers": ["ie >= 11"]
}
}]
],
"plugins": [
"transform-es2015-modules-commonjs",
"transform-class-properties"
]
}
webpack.common.js
const path = require('path')
const ExtractTextPlugin = require('extract-text-webpack-plugin')
const HtmlWebpackPlugin = require('html-webpack-plugin')
const cleanWebpackPlugin = require('clean-webpack-plugin')
const baseSCSS = new ExtractTextPlugin('main/_base.css')
const themeSCSS = new ExtractTextPlugin('main/_theme.css')
module.exports = {
entry: {
example: [
path.join(__dirname, 'src', 'example', 'index.js')
],
main: [
'idempotent-babel-polyfill',
path.join(__dirname, 'src', 'index.js')
]
},
output: {
path: path.join(__dirname, 'dist'),
filename: path.join('[name]', 'index.js')
},
module: {
rules: [
{
test: /\.js$/,
exclude: /node_modules/,
use: {
loader: 'babel-loader',
}
},
{
test: /\.scss$/,
use: ExtractTextPlugin.extract(
{
fallback: 'style-loader',
use: ['css-loader', 'sass-loader']
}
)
},
{
test: /\_base-scss$/,
use: baseSCSS.extract(
{
fallback: 'style-loader',
use: ['css-loader', 'sass-loader']
}
)
},
{
test: /\_theme-scss$/,
use: themeSCSS.extract(
{
fallback: 'style-loader',
use: ['css-loader', 'sass-loader']
}
)
}
]
},
plugins: [
new cleanWebpackPlugin('dist', {}),
new ExtractTextPlugin({ filename: path.join('example', 'style.css') }),
baseSCSS,
themeSCSS,
new HtmlWebpackPlugin({
inject: false,
hash: true,
template: path.join(__dirname, 'src', 'example', 'index.html'),
filename: path.join('example', 'index.html')
})
]
}
webpack.prod.js
const merge = require('webpack-merge')
const UglifyJSPlugin = require('uglifyjs-webpack-plugin')
const webpack = require('webpack')
const common = require('./webpack.common.js')
module.exports = merge(common, {
plugins: [
new UglifyJSPlugin({
sourceMap: true
}),
new webpack.DefinePlugin({
'process.env.NODE_ENV': JSON.stringify('production')
})
],
mode: 'production'
})
package.json
{
"name": "my-module-name",
"version": "1.0.0-beta.1",
"description": "",
"main": "dist/main/index.js",
"scripts": {
"start": "webpack-dev-server --config webpack.dev.js",
"server": "node src/server",
"format": "prettier-standard 'src/**/*.js'",
"lint": "eslint src",
"build": "webpack --config webpack.prod.js",
"test": "echo \"Error: no test specified\" && exit 1"
},
"author": "Liran",
"license": "ISC",
"devDependencies": {
"babel-core": "^6.26.0",
"babel-eslint": "^8.2.3",
"babel-loader": "^7.1.4",
"babel-plugin-transform-class-properties": "^6.24.1",
"babel-plugin-transform-es2015-modules-commonjs": "^6.26.2",
"babel-preset-env": "^1.7.0",
"clean-webpack-plugin": "^0.1.19",
"css-loader": "^0.28.11",
"eslint": "^4.19.1",
"extract-text-webpack-plugin": "^4.0.0-beta.0",
"html-webpack-plugin": "^3.2.0",
"idempotent-babel-polyfill": "^0.1.1",
"node-sass": "^4.9.0",
"prettier-standard": "^8.0.1",
"sass-loader": "^7.0.1",
"style-loader": "^0.21.0",
"uglifyjs-webpack-plugin": "^1.2.5",
"webpack": "^4.6.0",
"webpack-cli": "^2.0.15",
"webpack-dev-middleware": "^3.1.3",
"webpack-dev-server": "^3.1.3",
"webpack-merge": "^4.1.2"
}
}
Any help/pointers would be greatly appreciated. If you need more information, please let me know.

If you are not the library author and are having a problem consuming another library, you may be seeing an error like this:
TypeError: [LIBRARY_NAME]__WEBPACK_IMPORTED_MODULE_3__ is not a constructor
If that's the case, you may be importing the library incorrectly in your code (it may be a problem with default exports). Double check the library docs for usage.
It may be as simple as changing this:
import Foo from 'some-library/Foo';
to this:
import { Foo } from 'some-library';

It is not working because it is missing libraryTarget and library properties. By doing that webpack know which format of module you would like to create, i.e: commonjs (module.exports) or es (export).
I would do something like:
...
output: {
path: path.join(__dirname, 'dist'),
filename: path.join('[name]', 'index.js'),
library: "my-library",
libraryTarget: "umd" // exposes and know when to use module.exports or exports.
},
...

Besides setting the libraryTarget, it may also be necessary to move the export in the JavaScript file to the default.
function MyClassName() {
...
}
export default MyClassName;
And then in the webpack configuration the library type umd ...
(Note that I have used the newer library.type instead the older libraryTarget (see https://webpack.js.org/configuration/output/#outputlibrarytarget).
const path = require('path');
module.exports = {
mode: "production",
entry: '../wherever/MyClassName.js',
output: {
library: {
name: "MyClassName",
type: "umd", // see https://webpack.js.org/configuration/output/#outputlibrarytype
export: "default", // see https://github.com/webpack/webpack/issues/8480
},
filename: 'MyClassName.min.js',
path: path.resolve(__dirname, '../wherever/target/')
},
optimization: {
minimize: true
}
};
The export default makes the class available in JavaScript like the file was embedded directly, i.e.,
<script type="text/javascript" src="MyClassName.min.js"></script>
<script type="text/javascript">
<!--
var myInstance = new MyClassName();
// -->
</script>
Disclaimer: I added this answer even though the original question is three years old by now. After encountering the "is not a constructor" issue, it took me hours of searching to find the default solution. And that was the second time, I searched and found it :D

Cf. David Calhoun's answer, if you run into this with a third-party library, you may be trying to import a CommonJS module as an ECMAScript module. The workaround there seems to be to use require instead of import, e.g., instead of
import { Foo } from 'bar'
you need to write
const Foo = require('bar')
(There may be a more elegant way to handle this, but this is what worked for me.)

For me, it was the cache issue. Just cleared the cookies, cache data and closed, reopened the browser. It worked.

In my case, the error was being caused in React when trying to invoke JS's built-in Error constructor, or in other words, basically when calling throw new Error("something").
On inspection of my code, I realised I had a component called Error in my project which was being imported into the same file. The name clash between this component and JS's built-in Error constructor was causing the error mentioned in the question.

In case something is using wepack 5 + babel 7
"webpack": "5.73.0",
"#babel/core": "7.4.4",
"#babel/preset-env": "7.4.4",
"babel-loader": "8.0.5",
AND want to use class instead function, this worked for me:
class Person {
constructor(fname, lname, age, address) {
this.fname = fname;
this.lname = lname;
this.age = age;
this.address = address;
}
get fullname() {
return this.fname +"-"+this.lname;
}
}
export default Person;
In my case .babelrc was not necesary

tl;dr
Make sure that you import properly through index files.
Explanation
For me, this error was caused by importing through index files. I had multiple directories with their index.ts files that exported all the files inside the directory. These index files were accumulated/reexported by a main index.ts file so everything can be imported through it.
src/
├── index.ts
├── module1/
│ ├── index.ts
│ ├── file1.ts
│ └── file2.ts
└── module2/
├── index.ts
├── file3.ts
└── file4.ts
In file4.ts I had an import like this:
import { file1Class, file2Class, file3Class } from "src";
I had to split it into two separate imports:
import { file1Class, file2Class } from "src/module1";
import { file3Class } from "src/module2";