每天推薦一個 GitHub 優質開源項目和一篇精選英文科技或編程文章原文，歡迎關注開源日報。交流QQ群：202790710；微博：https://weibo.com/openingsource；電報群 https://t.me/OpeningSourceOrg

今日推薦開源項目：《甚至命令行畫圖 Termgraph》傳送門：GitHub鏈接

推薦理由：命令行是神通廣大的，你不僅可以拿來輸命令，還能拿來畫圖表。這個 Python 命令行工具可以讓你畫一些簡單的諸如條形圖這樣的圖表，照這個趨勢發展下去，遲早會有更高級的圖表可以拿命令行來畫……順帶一提，這個工具可以讓你皮一點——拿表情去充當條形圖的色塊玩，大概可以玩出這麼個效果，雖然圖不太准了，但是皮這一下還是很重要的。

今日推薦英文原文：《Learn the basics of the JavaScript module system and build your own library》作者：Kamlesh Chandnani

原文鏈接：https://medium.freecodecamp.org/anatomy-of-js-module-systems-and-building-libraries-fadcd8dbd0e

推薦理由：JS 模塊，也就是現在常見的一大堆的庫，這篇文章就講了關於它們的一些知識

Learn the basics of the JavaScript module system and build your own library

Lately we all have been hearing a lot about 「JavaScript Modules」. Everyone is likely wondering what to do with them, and how do they even play a vital role in our daily lives…?

So what the heck is the JS module system? ?

As JavaScript development gets more and more widespread, namespaces and dependencies get much more difficult to handle. Different solutions have been developed to deal with this problem in the form of module systems.

Why is understanding the JS Module System important?

Let me tell you a story.

Telling stories is as basic to human beings as eating. More so, in fact, for while food makes us live, stories are what make our lives worth living - Richard Kearney

Why am I even talking about all this stuff?

So, my day job is to design and architect projects, and I quickly realized that there were many common functionalities required across projects. I always ended up copy-pasting those functionalities to new projects over and over again.

The problem was that whenever one piece of the code changed, I needed to manually sync those changes across all my projects. To avoid all these tedious manual tasks, I decided to extract the common functionalities and compose an npm package out of them. This way, others on the team would be able to re-use them as dependencies and simply update them whenever a new release was rolled out.

This approach had some advantages:

• If there was some change in the core library, then a change only had to be made in one place without refactoring all the applications』 code for the same thing.
• All the applications remained in sync. Whenever a change was made, all the applications just needed to run the 「npm update」 command.

So, next step was to publish the library. Right? ?

This was the toughest part, because there were a bunch of things bouncing around in my head, like:

1. How do I make the tree shakeable?
2. What JS module systems should I target (commonjs, amd, harmony).
3. Should I transpile the source?
4. Should I bundle the source?
5. What files should I publish?

Everyone of us has had these kind of questions bubbling in our heads while creating a library. Right?

I』ll try to address all the above questions now.

Different Types of JS Module Systems ?

1. CommonJS

• Implemented by node
• Used for the server side when you have modules installed
• No runtime/async module loading
• import via 「require」
• export via 「module.exports」
• When you import you get back an object
• No tree shaking, because when you import you get an object
• No static analyzing, as you get an object, so property lookup is at runtime
• You always get a copy of an object, so no live changes in the module itself
• Poor cyclic dependency management
• Simple Syntax

// File log.js
function log(){
  console.log('Example of CJS module system');
}
// expose log to other modules
module.exports = { log }

// File index.js
var logModule = require('./log');
logModule.log();

2. AMD: Async Module Definition

• Implemented by RequireJs
• Used for the client side (browser) when you want dynamic loading of modules
• Import via 「require」
• Complex Syntax

// File log.js
define(['logModule'], function(){
    // export (expose) foo for other modules
    return {
        log: function(){
          console.log('Example of AMD module system');    
      }
    };
 });

// File index.js
require(['log'], function (logModule) {
  logModule.log();
});

3. UMD: Universal Module Definition

• Combination of CommonJs + AMD (that is, Syntax of CommonJs + async loading of AMD)
• Can be used for both AMD/CommonJs environments
• UMD essentially creates a way to use either of the two, while also supporting the global variable definition. As a result, UMD modules are capable of working on both client and server.

// File log.js
(function (global, factory) {
  if (typeof define === "function" && define.amd) {
    define(["exports"], factory);
  } else if (typeof exports !== "undefined") {
    factory(exports);
  } else {
    var mod = {
      exports: {}
    };
    factory(mod.exports);
    global.log = mod.exports;
  }
})(this, function (exports) {
  "use strict";

  function log() {
    console.log("Example of UMD module system");
  }
  // expose log to other modules
  exports.log = log;
});

4. ECMAScript Harmony (ES6)

• Used for both server/client side
• Runtime/static loading of modules supported
• When you import, you get back bindings value (actual value)
• Import via 「import」 and export via 「export」
• Static analyzing — You can determine imports and exports at compile time (statically) — you only have to look at the source code, you don』t have to execute it
• Tree shakeable, because of static analyzing supported by ES6
• Always get an actual value so live changes in the module itself
• Better cyclic dependency management than CommonJS

// File log.js
const log = () => {
  console.log('Example of ES module system');
}
export default log

// File index.js
import log from "./log"
log();

So now you know all about different types of JS module systems and how they have evolved.

Although the ES Harmony module system is supported by all the tools and modern browsers, we never know when publishing libraries how our consumers might utilize them. So we must always ensure that our libraries work in all environments.

Let』s dive in deeper and design a sample library to answer all the questions related to publishing a library in the proper way.

I』ve built a small UI library (you can find the source code on GitHub), and I』ll share all my experiences and explorations for transpiling, bundling, and publishing it.

Here we have a small UI library which has 3 components: Button, Card, and NavBar. Let』s transpile and publish it step by step.

Best practices before publishing ?

1. Tree Shaking ?

• Tree shaking is a term commonly used in the context of JavaScript for dead-code elimination. It relies on the static structure of ES2015 module syntax, that is, import and export. The name and concept have been popularized by the ES2015 module bundler rollup.
• Webpack and Rollup both support Tree Shaking, meaning we need to keep certain things in mind so that our code is tree shakeable.

// File shakebake.js
const shake = () => console.log('shake');
const bake = () => console.log('bake');

//can be tree shaken as we export as es modules
export { shake, bake };

// File index.js
import { shake } from './shakebake.js'
// only shake is included in the output

// File shakebake.js
const shake = () => console.log('shake');
const bake = () => console.log('bake');

//cannot be tree shaken as we have exported an object
export default { shake, bake };

// File index.js
import { shake } from './shakebake.js'
// both shake and bake are included in the output

// package.json

{
"name": "js-module-system",
"version": "0.0.1",
...

"main": "dist/index.js",
"module": "dist/index.es.js",

...
}

After looking at the build output for different libraries/packages, I got a clear picture of what different strategies the authors of these libraries might have had in mind before publishing. Below are my observations.

As you can see in the above image, I』ve divided these libraries/packages into two groups based on their characteristics:

1. UI Libraries (styled-components, material-ui)
2. Core Packages (react, react-dom)

If you』re a good observer ? you might have figured out the difference between these two groups.

UI Libraries have a dist folder that has the bundled and minified version for ES and UMD/CJS module systems as a target. There is a lib folder that has the transpiled version of the library.

Core Packages have just one folder which has the bundled and minified version for CJS or UMD module system as a target.

But why is there a difference in build output of UI libraries and Core Packages? ?

UI Libraries

Imagine if we just publish the bundled version of our library and host it on CDN. Our consumer will use it directly in a<script/> tag. Now if my consumer wants to use just the <Button/> component, they have to load the entire library. Also, in a browser, there is no bundler which will take care of tree shaking, and we』ll end up shipping the whole library code to our consumer. We don』t want this.

<script type="module">
import {Button} from "https://unpkg.com/uilibrary/index.js";
</script>

Now if we simply transpile the src into lib and host the lib on a CDN, our consumers can actually get whatever they want without any overhead. 「Ship less, load faster」. ✅

<script type="module">
import {Button} from "https://unpkg.com/uilibrary/lib/button.js";
</script>

Core Packages

Core packages are never utilized via the <script/> tag, as they need to be part of main application. So we can safely release the bundled version (UMD, ES) for these kinds of packages and leave the build system up to the consumers.

For example, they can use the UMD variant but no tree shaking, or they can use the ES variant if the bundler is capable of identifying and getting the benefits of tree shaking.

// CJS require
const Button = require("uilibrary/button");

// ES import
import {Button} from "uilibrary";

But…what about our question: should we transpile (Babelify) the source or bundle it? ?

For the UI Library, we need to transpile the source with Babel with the es module system as a target, and place it in lib. We can even host the lib on a CDN.

We should bundle and minify the source using rollup for cjs/umd module system and es module system as a target. Modify the package.json to point to the proper target systems.

// package.json

{
"name": "js-module-system",
"version": "0.0.1",
...

"main": "dist/index.js",      // for umd/cjs builds
"module": "dist/index.es.js", // for es build

...
}

For core packages, we don』t need the lib version.

We just need to bundle and minify the source using rollup for cjs/umd module system and es module system as a target. Modify the package.json to point to the proper target systems, same as above.

Tip: We can host the dist folder on the CDN as well, for the consumers who are willing to download the whole library/package via <script/> tag.

How should we build this?

We should have different scripts for each target system in package.json . You can find the rollup config in the GitHub repo.

// package.json

{
...
"scripts": {
"clean": "rimraf dist",
"build": "run-s clean && run-p build:es build:cjs build:lib:es",
"build:es": "NODE_ENV=es rollup -c",
"build:cjs": "NODE_ENV=cjs rollup -c",
"build:lib:es": "BABEL_ENV=es babel src -d lib"
}
...
}

What should we publish?

• License
• README
• Changelog
• Metadata(main , module, bin) — package.json
• Control through package.json files property

In package.json , the "files" field is an array of file patterns that describes the entries to be included when your package is installed as a dependency. If you name a folder in the array, then it will also include the files inside that folder.

We will include the lib and dist folders in "files" field in our case.

// package.json

{
...
"files": ["dist", "lib"]
...
}

Finally the library is ready to publish. Just type the npm run build command in the terminal, and you can see the following output. Closely look at the dist and lib folders. ?

Wrap up

Wow! Where does the time go? That was a wild ride, but I sincerely hope it gave you a better understanding of the JavaScript Module system and how you can create your own library and publish it.

Just make sure you take care of the following things:

1. Make it Tree Shakeable. ?
2. Target at least ES Harmony and CJS module systems. ?
3. Use Babel and Bundlers for libraries. ?
4. Use Bundlers for Core packages. ?
5. Set the module field of package.json to point to the ES version of your module (PS: It helps in tree shaking). ?
6. Publish the folders which have transpiled as well as bundled versions of you module. ?

Trending this week ?

1. Webpack-V4 alpha released. ?
2. ParcelJs: Blazing fast, zero configuration web application bundler. ?
3. Turbo: 5x faster than Yarn & NPM, and runs natively in-browser ?

每天推薦一個 GitHub 優質開源項目和一篇精選英文科技或編程文章原文，歡迎關注開源日報。交流QQ群：202790710；微博：https://weibo.com/openingsource；電報群 https://t.me/OpeningSourceOrg