module: remove --experimental-default-type

[GeoffreyBooth]

This PR removes --experimental-default-type per the discussion and consensus at the November 2024 Dublin collaborator summit. This also follows the discussion in nodejs/TSC#1445 and related issues where it was decided that Node.js would not be flipping the default module system anytime soon, if ever, now that the use case of “run ESM syntax without needing to opt in” has been satisfied by module syntax detection. cc @nodejs/loaders

[nodejs-github-bot]

Review requested:

• @nodejs/loaders
• @nodejs/test_runner

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63820/

[codecov]

Codecov Report

Attention: Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.00%. Comparing base (4ee87b8) to head (2e91784).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/modules/esm/get_format.js	87.09%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #56092 +/- ## ======================================= Coverage 88.00% 88.00% ======================================= Files 656 656 Lines 189000 188927 -73 Branches 35995 35961 -34 ======================================= - Hits 166320 166265 -55  + Misses 15840 15827 -13  + Partials 6840 6835 -5 

Files with missing lines	Coverage Δ
lib/internal/main/check_syntax.js	100.00% <100.00%> (+1.26%)	⬆️
lib/internal/main/eval_stdin.js	100.00% <100.00%> (ø)
lib/internal/main/eval_string.js	79.66% <100.00%> (-0.34%)	⬇️
lib/internal/main/run_main_module.js	100.00% <100.00%> (ø)
lib/internal/modules/esm/formats.js	98.55% <100.00%> (ø)
lib/internal/modules/esm/load.js	90.12% <100.00%> (-0.17%)	⬇️
lib/internal/modules/run_main.js	97.60% <100.00%> (+0.29%)	⬆️
lib/internal/process/execution.js	98.79% <100.00%> (ø)
src/node_options.cc	87.87% <ø> (+0.32%)	⬆️
src/node_options.h	98.29% <ø> (ø)
... and 1 more

... and 23 files with indirect coverage changes

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/63821/

[guybedford]

While I understand this option is deemed unnecessary for now, from the notes I was unable to determine the underlying reasons.

Specifically, has it been decided that Node.js should never implement a flag to change the top-level interpretation?

And if not, why should we remove this now, and what would have to change in future, to reconsider such a flag?

Anyone coming back to this issue will probably want to know the answers to these questions, so it would be very useful to clearly state for reference.

[GeoffreyBooth]

Specifically, has it been decided that Node.js should never implement a flag to change the top-level interpretation?

No formal decision has been made; landing this PR would be effectively making the decision, but only until someone lands a PR to add the flag back. Decisions are never final. If you'd like more discussion at the TSC level perhaps we could add this PR to the TSC agenda and you could join the next TSC meeting?

We know that flipping the default module system would be a breaking change to at least some (probably large) percentage of users, even years from now. I think the main question is: how would ESM-by-default be meaningfully different than the current status quo of syntax detection enabled by default, that would justify the disruption?

I can't really think of any differences that users would notice, other than ambiguous ESM JavaScript loading without a warning to add the type field; but that doesn't feel worth the disruption to me. The other use case would be running extensionless Wasm without a type field, but I'm unaware of that being something that anyone is doing; and we can support that without flipping the defaults if we really want to. Extensionless Wasm should currently throw when used as the entry point without --experimental-default-type=module, so it wouldn't be a breaking change to start running it.

And if not, why should we remove this now, and what would have to change in future, to reconsider such a flag?

The reasons to remove this are:

1. It adds a meaningful amount of maintenance burden to the modules codebase; 29 files changed and 800 lines of code, per this PR.

2. It also adds a lot of potential user confusion to the modules docs; there are many places where we need to mention that the described behavior is inverted if --experimental-default-type=module is passed.

3. Syntax detection satisfies the same core use cases as --experimental-default-type, of being able to run ESM syntax without needing to opt-in somehow, without being a breaking change. And importantly, syntax detection only works in one direction: we can try to run as CommonJS first and retry as ESM on parse error, but trying to run as ESM first wouldn't generate a parse error for typical CommonJS code. (Something like node --input-type=module --eval 'const fs = require("fs")' throws a ReferenceError, a runtime error, rather than the ParseErrors that we look for in syntax detection. We can't use runtime errors in syntax detection because then we're not just parsing code twice, but potentially running it twice.) So I think that flipping the default probably means also getting rid of syntax detection, and therefore abandoning the ability to run either module system's code without needing an opt-in somehow. Extensionless JavaScript files would only work when written as ESM, for example, not as either ESM or CommonJS as they do now.

As for what would have to change in the future, in the various discussions (sorry, I can't find links) there was talk about potentially revisiting this and flipping the default several years from now, once the transition to ESM is truly far along and not just writing but also running CommonJS syntax is anachronistic; but per https://github.com/wooorm/npm-esm-vs-cjs#data we're still at something like two-thirds of most popular npm libraries being CommonJS, with movement of only about 4% in the past year. At this rate it will be many years before we pass the tipping point, though hopefully require(esm) will accelerate things a bit. We can always put the flag back in when that occurs, along with a plan for actually going through with the flip. I don't think we should keep the flag around for the next five years or so in the meantime, as another experimental zombie that's neither removed nor moving toward stability. We already have too many of those.

[mcollina]

lgtm 🎉

[JakobJingleheimer]

One day, maybe :)

[nodejs-github-bot]

Landed in 94c327c

[joyeecheung]

Belated LGTM but I agree with #56092 (comment) - hopefully with require(esm) bridging migration the ESM tipping point will happen earlier than WW3, we'll see.