[experiment] implement tc39 Defer Module Evaluation #16567

Jack-Works · 2022-12-14T14:53:55Z

Does this PR introduce a breaking change?

no?

What needs to be documented once your changes are merged?

Defer import

This implements the semantics of the TC39 proposal Deferring Module Evaluation.

New options:

`experiments.deferImport: true`

To use the native syntax, install acorn-import-defer as a peer dependency.

// index.mjs
import defer * as ns from './mod.mjs'

function some_work() {
    console.log('deferred import value', ns.f)
}
console.log('entry runs')
setTimeout(some_work, 1000)

// mod.mjs
export function f() {
}
console.log('init cost!')

Result:

entry runs
// 1000ms later
init cost!
deferred import value, function f() {}

Other things to notice

Alternative syntax

import defer * as ns from './mod.mjs'

can be written as

import * as ns from /* webpackDefer: true */ './mod.mjs'

I suggest putting the magic comment after the from keyword, which is compatible with TypeScript, Babel, SWC, and Flow if the necessary option to preserve comments is enabled.

Please do not put it after the "import" keyword (like import /* webpackDefer: true */ * as ns from './mod.mjs'), the filesystem cache is broken, and I don't know why.

ESBuild is not compatible, please read evanw/esbuild#1439 and evanw/esbuild#309. Esbuild may support this feature ad-hoc like other webpack magic comments.

alexander-akait · 2022-12-14T15:06:24Z

Yeah, we can implement this under the experimental flag, feel free to ping when it's ready

webpack-bot · 2022-12-14T15:06:28Z

For maintainers only:

This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
This needs to be backported to webpack 4 (issue will be created when merged)

alexander-akait · 2022-12-18T15:33:26Z

Just update snaphots (-u):

FAIL test/StatsTestCases.basictest.js (11.925 s, 87 MB heap size)
  ● Console

alexander-akait · 2022-12-18T15:34:00Z

Test suite failed to run

Unable to find JEST_STATE_SYMBOL in Symbol(jest-native-promise), Symbol(jest-native-now), Symbol(jest-native-read-file), Symbol(jest-native-write-file), Symbol(jest-native-exists-file), Symbol($$jest-matchers-object), Symbol(graceful-fs.queue)

Is it locally?

alexander-akait · 2022-12-19T15:36:43Z

@Jack-Works Sounds like a bug in jest... do you use the same steps as in our CI file? Based on CI report you just need to update snapshot, no more

alexander-akait · 2022-12-20T11:31:14Z

Can you update StatsTestCases using -u flag for jest, thank you

alexander-akait · 2022-12-20T17:06:22Z

@Jack-Works What is node version? You can try to change, also just check you have the same versions as in the lock file

alexander-akait · 2022-12-21T13:26:10Z

Looks like 18.0.0 is the first breaking Node version. I don't know what happened exactly.

Weird, I don't have problem with jest locally... Okay, we can try to update jest (maybe new versions resolve this problem)

Also can you use Node@16 to update snapshots?

Jack-Works · 2022-12-24T03:50:24Z

I tried to add some test cases (6041d87) but it seems like does not work... Did I do right?

alexander-akait · 2022-12-27T11:08:51Z

That will be a little pain, I need to adjust my setup (installing nvm-for-windows seems to require uninstalling all npm globals and the current node install), I'll use this as the last solution joy

You can use virtual machine here 😄

codspeed-hq · 2025-05-02T16:07:19Z

CodSpeed Performance Report

Merging #16567 will degrade performances by 82.72%

_{Comparing Jack-Works:lazy-import (2d3b95c) with main (d4f275d)}

Summary

⚡ 1 improvements
❌ 3 regressions
✅ 129 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	`BASE`	`HEAD`	Change
⚡	`benchmark "devtool-eval-source-map", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}'`	43.2 ms	10.3 ms	×4.2
❌	`benchmark "future-defaults", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}'`	10 ms	52.6 ms	-81.08%
❌	`benchmark "lodash", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}'`	27.4 ms	34.8 ms	-21.25%
❌	`benchmark "many-modules-esm", scenario '{"name":"mode-development-rebuild","mode":"development","watch":true}'`	11.2 ms	64.7 ms	-82.72%

codspeed-hq · 2025-05-02T16:08:07Z

CodSpeed Performance Report

Merging #16567 will degrade performances by 23.26%

_{Comparing Jack-Works:lazy-import (de9c023) with main (3f36f91)}

Summary

❌ 1 regressions
✅ 44 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	`BASE`	`HEAD`	Change
❌	`benchmark "typescript-long", scenario '{"name":"mode-development","mode":"development"}'`	27.8 s	36.2 s	-23.26%

nicolo-ribaudo · 2025-05-02T18:12:53Z

The same test becomes 23% slower in development and 23% faster in production? 🤔

Jack-Works · 2025-05-11T06:18:09Z

hi @alexander-akait I think it is ready now!

alexander-akait · 2025-06-02T16:12:07Z

lib/dependencies/ImportParserPlugin.js

@@ -46,6 +46,8 @@ class ImportParserPlugin {
 			Array.from(enumerable, e => [e]);
 		parser.hooks.importCall.tap("ImportParserPlugin", expr => {
 			const param = parser.evaluateExpression(expr.source);
+			// TODO support deferImport
+			// this requires the evaluateExpression to support Object literals.


@Jack-Works So we don't support import('something', { defer: true })? Do we have this in spec?

@Jack-Works So we don't support import('something', { defer: true })? Do we have this in spec?

No, the syntax becomes import.defer('something') (this comment is written in a very early stage), I'll update this soon.

lib/javascript/JavascriptParser.js

+		 * @this {TODO}
+		 * @param {TODO} node node
+		 * @returns {TODO} import.meta
+		 */


alexander-akait

@Jack-Works Sorry for delay, there are two question, and we can merge (want to merge it this week), thank you for your work

Co-authored-by: Nicolo Ribaudo <hello@nicr.dev>

Jack-Works · 2025-06-04T09:34:27Z

updated, but import.defer() is marked as not implemented. I may implement it in the future

alexander-akait · 2025-06-04T12:15:56Z

@Jack-Works Just for information - any blockers for import.defer('something')?

Jack-Works · 2025-06-04T16:52:53Z

@Jack-Works Just for information - any blockers for import.defer('something')?

Not yet, I didn't do any research on this because I'm focusing on other things right now. I emit this error to make the PR merge-ready (without a breaking import.defer that acts as import).

nicolo-ribaudo · 2025-06-09T10:17:26Z

Fyi, TypeScript just merged support for this :)

alexander-akait · 2025-06-10T21:15:36Z

lib/runtime/MakeDeferredNamespaceObjectRuntime.js

+				])},`,
+				`defineProperty: ${runtimeTemplate.basicFunction("_, name", [
+					init,
+					// TODO:


What we should do here in future?

A Module Namespace object's defineProperty checks the new property descriptor. It only returns true when the new property descriptor is the same as the current one.

This behaviour is skipped since webpack's implementation (lib/runtime/MakeNamespaceObjectRuntimeModule.js) for normal namespace object is skipping this too.

alexander-akait · 2025-06-10T21:16:15Z

test/configCases/defer-import/comment-error/webpack.config.js

+		environment: {
+			optionalChaining: false
+		}
+	},


Very weird, I will resolve it before merge, no need action from you

lib/runtime/MakeDeferredNamespaceObjectRuntime.js

lib/javascript/JavascriptModulesPlugin.js

lib/runtime/MakeDeferredNamespaceObjectRuntime.js

webpack-bot added the PR: unreviewed label Dec 14, 2022

Jack-Works force-pushed the lazy-import branch 2 times, most recently from 7420ab0 to 32408f7 Compare December 16, 2022 14:21

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Jack-Works force-pushed the lazy-import branch 2 times, most recently from 9161b0f to 5250fd6 Compare March 23, 2023 11:09

webpack-bot removed the PR: conflict label Mar 23, 2023

Jack-Works force-pushed the lazy-import branch from 5250fd6 to a16aae9 Compare March 23, 2023 11:15

Jack-Works force-pushed the lazy-import branch from de9c023 to db2bd21 Compare May 2, 2025 14:54

alexander-akait reviewed Jun 2, 2025

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lib/javascript/JavascriptParser.js

* @this {TODO}

* @param {TODO} node node

* @returns {TODO} import.meta

*/

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alexander-akait reviewed Jun 2, 2025

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Jack-Works force-pushed the lazy-import branch from db2bd21 to afc3084 Compare June 4, 2025 08:49

feat: implement tc39 defer import proposal

9b5df20

Co-authored-by: Nicolo Ribaudo <hello@nicr.dev>

Jack-Works force-pushed the lazy-import branch from afc3084 to 9b5df20 Compare June 4, 2025 08:53

Boshen mentioned this pull request Jun 7, 2025

[Feature Request]: import defer rolldown/rolldown#4857

Open

chenjiahan mentioned this pull request Jun 9, 2025

[Feature]: Support for Deferring Module Evaluation (import defer) web-infra-dev/rspack#10608

Open

alexander-akait approved these changes Jun 10, 2025

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Jack-Works and others added 3 commits June 26, 2025 00:55

fix: keys of DeferredNamespaceObject

741514a

Merge remote-tracking branch 'origin/main' into lazy-import

4af610f

refactor: MakeDeferredNamespaceObjectRuntimeModule

2d3b95c

hai-x force-pushed the lazy-import branch from 22c2f59 to 2d3b95c Compare July 1, 2025 16:04

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[experiment] implement tc39 Defer Module Evaluation #16567

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[experiment] implement tc39 Defer Module Evaluation #16567

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Jack-Works commented Dec 14, 2022 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Defer import

New options:

experiments.deferImport: true

Other things to notice

Alternative syntax

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alexander-akait commented Dec 14, 2022

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alexander-akait commented Dec 18, 2022

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alexander-akait commented Dec 20, 2022 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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codspeed-hq bot commented May 2, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Merging #16567 will degrade performances by 82.72%

Summary

Benchmarks breakdown

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codspeed-hq bot commented May 2, 2025

Merging #16567 will degrade performances by 23.26%

Summary

Benchmarks breakdown

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nicolo-ribaudo commented May 2, 2025

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Jack-Works commented May 11, 2025

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Jack-Works commented Jun 4, 2025

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Jack-Works commented Dec 14, 2022 •

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`experiments.deferImport: true`

alexander-akait commented Dec 20, 2022 •

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