Edit (06/01/21): Thanks to the Oclif team for letting me contribute ESM support to Oclif v2! I have updated all links in this issues comments to be as accurate as possible. Please see this comment on a current workaround to publish an ESM CLI while the rest of the Oclif v2 infrastructure is updated.

Hi Philipe & Thomas (@RasPhilCo / @amphro) et al,

Great work on Oclif v2 Philipe and all involved. I really like the refactor to a core module compared to v1. Removing the circular dependencies (IE between v1 plugin-help and plugin-command) helps a bunch. I would like to discuss adding ES Module (ESM) support to Oclif v2 before initial launch in ~June. I initially prototyped the changes in v1 and saw the core announcement and have fully worked out the essential changes to @oclif/core for v2. I have a fork of @oclif/core with these changes and have fully implemented non-trivial Oclif middleware and resulting CLI w/ ESM to test everything out that is published on NPM using my @oclif/core fork. I also have three test repositories that have stripped down CLI tests with variations that respectively cover a wide swath of the 12.0.0 to 15.x Node ecosystem by Github Actions. I'll comment on this below in another post.

This kind of fundamental change is best in a new version of Oclif and is timely due to the minimum Node support of v12 for Oclif v2 which is the first Node LTS version that has wider support for ES Modules. Supporting ESM now future proofs Oclif as the larger Node ecosystem transitions to ESM in the coming years. There is a fair amount of movement in this direction already in April / May as Node 12 becomes the minimum LTS release. These changes do not affect current support of CommonJS / Typescript CLIs. In fact there could be some benefit for end user CLI development in Typescript as one could target ESM instead of CJS and if launching a CLI w/ Node 14 as a minimum version for support can target ES2020 for Typescript or simply write the CLI in ESM / ES2020. The main core Oclif v2 code should still of course target CJS for now, but this frees up end users to choose their target of choice between CJS / ESM / Typescript.

I will go over in more detail in a follow up post to this issue regarding what the changes cover and note the other areas of the larger codebase / other modules that require changes. I have already gone through several iterations of improving the essential changes and have arrived at what I think is a clean addition which I have tested across all the OS platforms w/ Node 12.0.0 - 15.x. Beyond just submitting the pull requests / changes I am fully committed to helping write new tests / verifying Node 12.0.0 compatibility and assisting with writing documentation changes for v2. IE not just provide a code dump and add extra burden to any internal team members before the v2 launch. I believe there is enough time to get these changes in responsibly before the v2 ~June launch.

Instead of just making a pull request to @oclif/core I'd like to discuss willingness to add ESM support through my involvement first. In follow up posts to this issue I'll discuss the essential changes to @oclif/core and other modules (plugin-help / dev-cli / oclif / test). I'm 100% open to modifying anything necessary before / while making any pull requests. I look forward to working with the Oclif team to support ESM and make Oclif v2 a very solid solution for advanced CLI development on Node now and into the future.