Shuhei Kagawa

How to export CommonJS and ES Module

Jan 5, 2017 - JavaScript

After my previous post about jsnext:main and module, there came another issue.

Here is the twists and turns that I wandered to solve the problem.


The code of material-colors looked like the following.

colors.js specified in main (CommonJS version)

module.exports = {
  red: { /* ... */ },
  blue: { /* ... */ }

colors.es2015.js specified in jsnext:main/module (ES Module version)

export var red = { /* ... */ };
export var blue = { /* ... */ };

Then the ES Module file can get benefit of tree shaking if it's imported by named imports.

Problem of having only named exports

The colors.es2015.js broke react-color when built with Webpack 2 because it was doing default import but colors.es2015.js didn't have default export.

import material from 'material-colors';

So @echenley suggested to change it to a wildcard import.

import * as material from 'material-colors';

It worked well, but I removed jsnext:main and module because other libraries with default import may break on Webpack 2 and material-colors is already tiny without tree shaking anyway.

Have a default export

After a while, I came up with a better solution to have a default export in addition to named exports. Then it will work well with tree shaking and won't break default import. Pretty obvious after coming up.

export var red = { /* ... */ };
export var blue = { /* ... */ };

export default {
  red: red,
  blue: blue


To keep maximum compatibility for CommonJS and ES Module:

  • If your CommonJS module exports only one thing, like encouraged in the npm world, export it as a default export.
  • If your CommonJS module exports multiple things, which essentially exports an object with them as properties, export named exports. In addition to it, it's safer to have a default export just in case for the problem described above.

main, jsnext:main and module

Jan 5, 2017 - JavaScript

Node module's package.json has main property. It's the entry point of a package, which is exported when a client requires the package.

Recently, I got an issue on one of my popular GitHub repos, material-colors. It claimed that "colors.es2015.js const not supported in older browser (Safari 9)", which looked pretty obvious to me. ES2015 is a new spec. Why do older browsers support it?

I totally forgot about it at the time, but the colors.es2015.js was exposed as the npm package's jsnext:main. And to my surprise, it turned out that jsnext:main shouldn't have jsnext or ES2015+ features like const, arrow function and class. What a contradiction!


Module bundlers that utilizes tree shaking to reduce bundle size, like Rollup and Webpack 2, require packages to expose ES Modules with import and export. So they invented a non-standard property called jsnext:main.

However, it had a problem. If the file specified jsnext:main contains ES2015+ features, it won't run without transpilation on browsers that don't support those features. But normally people don't transpile packages in node_modules, and many issues were created on GitHub. To solve the problem, people concluded that jsnext:main shouldn't have ES2015+ features other than import and export. What an irony.


Now the name jsnext:main is too confusing. I was confused at least. People discussed for a better name, and module came out that supersedes jsnext:main. And it might be standardized.


I looked into a couple of popular repos, and they had both of jsnext:main and module in addition to main.

At this time, it seems to be a good idea to have both of them if you want to support tree shaking. If you don't, just go with only the plain old main.

How to set up and top up a prepaid SIM in Germany

Oct 3, 2016

I have moved to Berlin from Tokyo a week ago. I may or may not write about it later, but I'm going to share more practical stuff today.

Since I arrived in Germany, I bought two prepaid SIM cards and set up a SIM-free iPhone and a MiFi (mobile WiFi router). It was harder than I thought because I had never used a prepaid SIM before and most of official instructions were in German. I'd like to share what I did for people like me.

This post is a complement to Germany | Prepaid Data SIM Card Wiki | Fandom powered by Wikia. If you haven't read it yet, read it first.

Initial setup

After reading the wiki, I chose O2 as a network provider because of its rate and availability in Berlin's subway.

  1. Go to a large electronics store like Saturn.
  2. Find an O2 prepaid SIM.
  3. Bring it to an O2 representative in the store and ask her to activate it.
  4. Go to the casher and pay for the SIM card.
  5. Insert the SIM card into your phone/MiFi. You can ask shop staffs to open SIM card slot.
  6. Unlock the SIM card. PIN is on the white card that contains the SIM card. Also its phone number (Rufnummer) is on the same card.

Topping up

You can top up your SIM via a call, O2 mobile app or O2 website. I used O2 website because I couldn't make a call with my MiFi and I don't have German AppStore's account. The website is only in German. So it's convenient to use Google Chrome's translation feature.

  1. Go to a drug store chain like dm and buy one of O2 top-up cards like €20. They are usually put next to other prepaid cards like Apple, Google Play, Amazon, Zalando, etc. The actual top-up code is printed on your receipt. Or you can buy a top-up code online at Thanks, Yan Yankowski for letting me know!
  2. Sign up for O2 website.
  3. Enter your phone number (Mobilfunknummer), preliminary password (Vorläufiges Kennwort) and new password (Neues Kennwort). The preliminary password is notified via SMS. If it's for your MiFi, you can access to the MiFi's admin page and read SMS.
  4. Go to Recharge tab (Mein O2 -> Mein Prepaid -> Guthaben & Aufladen) and enter your top-up card's code.
  5. (Optional) Choose your favorite plan (Tarif & SIM-Karte).