JavaScript 世界的模块系统

• The JavaScript language didn’t have a native way of organizing code before the ES2015 standard.
• Node.js filled this gap with the CommonJS module format.
• The module system allows you to organize your code, hide information and only expose the public interface of a component using module.exports. Every time you use the require call, you are loading another module.
• do not use exports = xxx directly, use module.exports = xxx insteadly.

Module is wrapped by Node.js this way

(function (exports, require, module, __filename, __dirname) {
  function add (a, b) {
    return a + b
  }

  module.exports = add
})

This is why you can access the global-like variables like require and module. It also ensures that your variables are scoped to your module rather than the global object.

How does require works

• The module loading mechanism in Node.js is caching the modules on the first require call.
• The module dealing with module loading in the Node core is called module.js, and can be found in lib/module.js in the Node.js repository.
• Does module.exports in node js create a shallow copy or deep copy of the exported objects or functions?

What’s in your node_modules?

• npm v2: npm 2 installs all dependencies in a nested way, where your primary package dependencies are in their node_modules folder.
• npm v3: npm3 attempts to flatten these secondary dependencies and install them in the root node_modules folder. This means that you can’t tell by looking at your node_modules which packages are your explicit or implicit dependencies.

ES2015

• As we saw above, the CommonJS module system uses a runtime evaluation of the modules, wrapping them into a function before the execution.
• The ES2015 modules don’t need to be wrapped since the import/export bindings are created before evaluating the module. This incompatibility is the reason that currently there are no JavaScript runtime supporting the ES modules.

The structure

• JS world
  • ECMAScript standards
    • ES5
    • ES6/ES2015
  • TypeScript
• Other information
  • ES5 is what most of us have used for years. Functional programming at its best, or worst, depending on how you view it. I personally love programming with ES5. All modern browsers support it.
  • ECMAScript 2015 is an ECMAScript standard that was ratified in June 2015.

How to enable ES2015 in nodejs

• How do we run ES2015 in browsers that do not yet support ES2015? We can use ES2015 and transpile to ES5 using a tool like Babel.

Other module system

• UMD

References

• How the module system, CommonJS & require works | @RisingStack
• javascript - What is the purpose of Node.js module.exports and how do you use it? - Stack Overflow
• Learn the basics of the JavaScript module system and build your own library
• Node.js, TC-39, and Modules – Hacker Noon
• export - Web technology for developers | MDN
• Other
  • History of Node.js on a Timeline | @RisingStack
  • Understanding the Node.js Event Loop | @RisingStack
  • Parcel 中文网