plan-for-new-modules-implementation.md

Plan for New Modules Implementation

This document outlines the plan for building a new implementation to support ECMAScript modules in Node.js. The general idea is to start with a “minimal kernel” as Phase 1, which consists of features that the @nodejs/modules group have agreed will be necessary for all potential iterations of our ESM implementation. Phase 1 does not include features that preclude other potential features or implementation approaches; and Phase 1 also does not include some features that should naturally be built in a later phase of development, for example because those features depend on features planned for Phase 1. The minimal kernel/phase 1 is not intended to be merged into Node core or released; it is only a starting point for gradually building layers of consensus.

At every phase, the following standards must be maintained:

• Spec compliance (#132): We must always follow the ES spec.
• Browser equivalence (#133): There’s room for debate in specific cases, but in general if Node is doing something that browsers also do, Node should do it in the same way. Alternatively, code that executes in both environments should produce identical results.
• Don’t break CommonJS (#112): We cannot cause breaking changes with regards to CommonJS.

See also the features list in the README.

Phase 1: The Minimal Kernel

These features will be part of the first phase of development:

• module.createRequireFromPath (nodejs/node#19360) is the only way to import CommonJS into an ES module, for now.

  • import.meta.require fails at runtime as opposed to import time. This is not desireable to all committee members
  • Hold off on import statements for CommonJS until more progress is made on the dynamic modules spec.
  • landed in https://github.com/nodejs/node/commit/246f6332e5a5f395d1e39a3594ee5d6fe869d622

• import statements will only support files with an .mjs extension, and will import only ES modules, for now.

  • In a later phase, the intention is to move forward with format databases to map extensions and support multiple use cases.
  • No JSON or native modules; createRequireFromPath can be used to get these.

• import.meta.url.

  • Already in the existing implementation.

• Dynamic import().

  • Already in the existing implementation.

• Support for built-in modules with named exports

  • Already in the existing implementation

How will we get from where we are to Phase 1

• Remove support in the import statement of formats other than ESM:

  • No CommonJS.
  • No JSON.
  • No native modules.

• Remove dynamic path searching:

  • No extension adding.
  • No directory resolution, including no support for index.js or index.mjs.
  • No support for main field for ESM.

• Remove current VM implementation

• Remove current Loader implementation

These changes are implemented in nodejs/ecmascript-modules#6

Phase 2

Phase 2 will focus on uncontentious features to enhance UX:

• improving cjs interop in esm
  • createRequireFromURL
  • tracking issue: REPLACEME
• Package.json meta data for bare imports
  • main vs module?
  • tracking issue: REPLACEME
• Re-Introduce VM module integration
  • tracking issue: REPLACEME
  • Implemented in: nodejs/ecmascript-modules#8

Phase 3

Phase 3 will focus on extensible "loaders" and deliver an environment that allows user-land experimentation.

Phase 4

Phase 4 will focus on addressing the user feedback gathered from the experimentation enabled by Phase 3.