esm: avoid `import.meta` setup costs for unused properties #57003

JakobJingleheimer · 2025-02-11T10:54:37Z

Currently, the cost of deriving import.meta.dirname and import.meta.filename is paid up-front, regardless of whether they are actually used. This can instead be delayed until they are accessed, and then to avoid re-paying the cost, overwrite the getter with the derived value.

nodejs-github-bot · 2025-02-11T10:54:42Z

Review requested:

@nodejs/loaders

jsumners-nr

For what it's worth, my personal time identity backed this out due to a lot of waffling and someone finally providing evidence against it -- #48740 (comment)

(Very easy to overlook that in all of the nonsense on that PR.)

codecov · 2025-02-11T12:43:14Z

Codecov Report

Attention: Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.27%. Comparing base (3186468) to head (c3c12a4).
Report is 546 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/modules/esm/initialize_import_meta.js	93.93%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57003      +/-   ##
==========================================
+ Coverage   89.13%   90.27%   +1.14%     
==========================================
  Files         665      630      -35     
  Lines      193125   186189    -6936     
  Branches    37182    36474     -708     
==========================================
- Hits       172136   168089    -4047     
+ Misses      13734    10979    -2755     
+ Partials     7255     7121     -134

Files with missing lines	Coverage Δ
lib/internal/modules/esm/initialize_import_meta.js	`98.16% <93.93%> (-1.84%)`	⬇️

... and 270 files with indirect coverage changes

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JakobJingleheimer · 2025-02-11T12:48:33Z

(Very easy to overlook that in all of the nonsense on that PR.)

😆

For what it's worth, my personal time identity backed this out due to a lot of waffling and someone finally providing evidence against it -- #48740 (comment)

It looks like the spec was misquoted.

I'm not sure what Antoine was getting at with

I don't get it though, the getters are evaluated right away, there's no point in keeping them or am I missing something?

Getters are not run right away—that's their whole point.

jsumners-nr · 2025-02-11T12:58:04Z

I don't get it though, the getters are evaluated right away, there's no point in keeping them or am I missing something?

Getters are not run right away—that's their whole point.

They were in that specific implementation. But I was quite happy to stop churning on the detail.

I am not against these being getters. I'm just linking the history.

ljharb · 2025-02-11T15:43:00Z

https://tc39.es/ecma262/#sec-meta-properties-runtime-semantics-evaluation

The spec’s intention is very much that import.meta only contains data properties. It’s surely possible to use C++ to make an object that appears to have data properties but avoids paying the up front cost, but it wouldnt be possible in JS.

As is, this change is observable to someone looking at property descriptors, and this i think needs to be semver major.

jkrems · 2025-02-11T16:44:21Z

https://tc39.es/ecma262/#sec-meta-properties-runtime-semantics-evaluation

The spec’s intention is very much that import.meta only contains data properties. It’s surely possible to use C++ to make an object that appears to have data properties but avoids paying the up front cost, but it wouldnt be possible in JS.

As is, this change is observable to someone looking at property descriptors, and this i think needs to be semver major.

I agree with Bradley in the other thread that HostFinalizeImportMeta does exist in that algorithm and gives the host/nodejs explicit permission to make other changes to the object, including adding non-data properties.

I don't think the exact property descriptor is "part of the API" here but I don't feel strongly enough about it to say that it must be semver minor over major.

lib/internal/modules/esm/initialize_import_meta.js

ljharb · 2025-02-11T16:48:57Z

It absolutely allows it - but the intention was for freezing and not for making getters, as i recall.

mcollina

lgtm

bmeck · 2025-02-11T22:40:44Z

@ljharb as I recall it was for freezing from Agoric side of convo and altering proto from web vendor side (including a potential desire for accessors from proto).

ljharb · 2025-02-11T22:41:45Z

ah, I don't recall that second part.

bmeck · 2025-02-11T22:45:04Z

@ljharb they never shipped it and likely never will at this point

bmeck · 2025-02-12T23:37:52Z

an alternative that is still a data property and is lazy is to use https://v8.github.io/api/head/classv8_1_1Object.html#a918a985db244fdc1bde268741e840620

ljharb

I still consider this a breaking change, but LGTM

JakobJingleheimer · 2025-04-22T20:47:29Z

Ah, damn. Looks like it will miss 24.0.0; I was thinking it still might land for that and breaking/non-breaking would be a discussion we don't have to have 😅 (aside from backport).

ljharb · 2025-04-22T20:50:06Z

Doing it in C++ would make it nonbreaking, if that's preferred.

JakobJingleheimer · 2025-04-22T20:54:56Z

Doing it in C++ would make it nonbreaking, if that's preferred.

Mmm, I was hoping/expecting that approach to tick all the boxes. But it seems a mixed bag, IMO leaning towards the less ideal.

I was just planning to get this PR to a landable state so then we can decide A or B.

To me, the approach in this PR is in the vicinity of breaking territory in that it may be observable. But, I'm thinking it won't actually be tangible (and thus won't actually break anything).

ljharb · 2025-04-22T20:56:50Z

I agree that breakage is unlikely, but semver doesn't deal in likelihoods :-)

fwiw it seems perfectly fine to land this on 25+, and then convert it to C++ in a future PR, at which point both commits would be maximally backportable together.

nodejs-github-bot · 2025-04-22T20:58:45Z

CI: https://ci.nodejs.org/job/node-test-pull-request/66416/

nodejs-github-bot · 2025-04-23T09:24:45Z

Commit Queue failed

- Loading data for nodejs/node/pull/57003
✔  Done loading data for nodejs/node/pull/57003
----------------------------------- PR info ------------------------------------
Title      esm: avoid `import.meta` setup costs for unused properties (#57003)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     JakobJingleheimer:esm/avoid-setup-costs-for-unused-importmeta-props -> nodejs:main
Labels     semver-major, esm, author ready, needs-ci, commit-queue-squash
Commits    3
 - esm: avoid `import.meta` setup costs for unused properties
 - !fixup: correct autocomplete of prop name
 - fixup!: add test case for re-assigning `dirname` & `filename`
Committers 1
 - Jacob Smith <[email protected]>
PR-URL: https://github.com/nodejs/node/pull/57003
Reviewed-By: Jan Martin <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Jordan Harband <[email protected]>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/57003
Reviewed-By: Jan Martin <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Jordan Harband <[email protected]>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Tue, 11 Feb 2025 10:54:37 GMT
   ✔  Approvals: 3
   ✔  - Jan Martin (@jkrems): https://github.com/nodejs/node/pull/57003#pullrequestreview-2785333954
   ✔  - Matteo Collina (@mcollina) (TSC): https://github.com/nodejs/node/pull/57003#pullrequestreview-2610219287
   ✔  - Jordan Harband (@ljharb): https://github.com/nodejs/node/pull/57003#pullrequestreview-2785323386
   ✘  semver-major requires at least 2 TSC approvals
   ✔  Last GitHub CI successful
   ℹ  Last Full PR CI on 2025-04-22T20:58:45Z: https://ci.nodejs.org/job/node-test-pull-request/66416/
- Querying data for job/node-test-pull-request/66416/
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--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/14614621512

JakobJingleheimer · 2025-04-23T10:41:13Z

Ah, needs 2 TSC approvals, not just 1.

aduh95 · 2025-04-25T07:15:55Z

Benchmark CI: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1694/

esm/esm-legacyMainResolve.js resolvedFile='node_modules/test/index.js' packageConfigMain='' packageJsonUrl='node_modules/test/package.json' n=10000                     **     -0.80 %       ±0.52%
esm/import-meta.js n=1000                                                                                                                                              ***     -9.01 %       ±0.81%

JakobJingleheimer · 2025-04-25T08:21:43Z

I don't know how to read that benchmark 😅

Could we also run CitGM? (I don't know how to trigger it)

aduh95 · 2025-04-25T08:37:06Z

The benchmark shows a worst result than #57286 for esm/import-meta.js, I guess that's not surprising, and is not a deal breaker I don't think. That being said, it'd probably be a good idea for me (or someone else) to go back to that other PR to see if where we could improve it as it would address Jordan concerns and be potentially better perf-wise

aduh95 · 2025-04-29T20:33:01Z

Benchmark CI to include the results form #58056: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1710/

                                                                     confidence improvement accuracy (*)
esm/esm-loader-import.js specifier='node:prefixed-nonexistent' n=1000     *     -3.67 %       ±3.56%
esm/import-meta.js valuesToRead='dirname-and-filename' n=1000           ***     -7.97 %       ±0.62%
esm/import-meta.js valuesToRead='dirname' n=1000                        ***     -7.81 %       ±0.82%
esm/import-meta.js valuesToRead='filename' n=1000                       ***     -6.14 %       ±0.83%

aduh95 · 2025-04-29T20:58:40Z

Superseded by #57286

nodejs-github-bot added esm Issues and PRs related to the ECMAScript Modules implementation. needs-ci PRs that need a full CI run. labels Feb 11, 2025

esm: avoid import.meta setup costs for unused properties

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JakobJingleheimer force-pushed the esm/avoid-setup-costs-for-unused-importmeta-props branch from 90d6aac to b3575a0 Compare February 11, 2025 11:34

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!fixup: correct autocomplete of prop name

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src: port initializeImportMeta to native #57286

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fixup!: add test case for re-assigning dirname & filename

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JakobJingleheimer force-pushed the esm/avoid-setup-costs-for-unused-importmeta-props branch from ed9b3cb to c3c12a4 Compare April 22, 2025 19:15

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