Intent to Ship: Deferred imports evaluation ("import defer")

[Caio Lima]

Contact emails

nrib...@igalia.com, caio...@igalia.com

Explainer

https://github.com/tc39/proposal-defer-import-eval

Specification

https://tc39.es/proposal-defer-import-eval

Summary

`import defer` allows lazily evaluating ES modules, to reduce the startup impact of modules that are not actually needed for the first JavaScript execution. This provides an easier-to-use alternative to dynamic import. While dynamic import provides more benefits, as it allows deferring loading of modules rather than just evaluation, it has a lot of friction due to its asynchronous nature, and thus it's not always usable. The `import defer` TC39 proposal follows the same syntax as `import source`, but it only works with namespace imports: ``` import defer * as namespace from "./some-module.js" ``` The module will be synchronously executed on property access on the module namespace object. To allow synchronous execution, not all of `./some-module.js`'s dependency tree will be deferred. Modules that contain top-level await will be eagerly executed, leaving only the synchronous parts of the tree for later.

Blink component

Blink>JavaScript

Web Feature ID

import-defer

Motivation

No information provided

Initial public proposal

No information provided

TAG review

No information provided

TAG review status

Not applicable

Goals for experimentation

None

Risks

Interoperability and Compatibility

No information provided

Gecko: Positive (https://bugzilla.mozilla.org/show_bug.cgi?id=1952263) Mozilla was originally championing the proposal

WebKit: Shipped/Shipping (https://bugs.webkit.org/show_bug.cgi?id=320867)

Web developers: No signals

Other signals:

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

No information provided

Debuggability

Changes were added to CDP to properly support feature UX (for detailed information, see https://issues.chromium.org/issues/552666773)

Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

No

Is this feature fully tested by web-platform-tests?

Yes
Test262 tests: - https://github.com/tc39/test262/tree/main/test/language/import/import-defer - https://github.com/tc39/test262/tree/main/test/language/expressions/dynamic-import/import-defer

Flag name on about://flags

No information provided

Finch feature name

V8Flag_js_defer_import_eval

Rollout plan

Will ship enabled for all users

Requires code in //chrome?

False

Tracking bug

https://issues.chromium.org/issues/398218423

Estimated milestones

Shipping on desktop	155
Shipping on Android	155
Shipping on WebView	155

Anticipated spec changes

Open questions about a feature may be a source of future web compat or interop issues. Please list open issues (e.g. links to known github issues in the project for the feature specification) whose resolution may introduce web compat/interop risk (e.g., changing to naming or structure of the API in a non-backward-compatible way).

No information provided

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5071131271888896?gate=6546259918848000

This intent message was generated by Chrome Platform Status.

[Dan Clark]

> Web developers: No signals

Is there any sign of developer interest we can point to here?

> Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

> No

I'd expect this to be shipping everywhere -- should this say "yes"?

-- Dan

[Caio Lima]

On Sep 14, 2026, at 6:12 PM, Dan Clark <dan...@microsoft.com> wrote:

> Web developers: No signals

I’ve found this documented signal on proposal’s repo https://github.com/tc39/proposal-defer-import-eval/issues/18. It’s important to notice that since it’s from 2023, it predates the `import defer` syntax, but the semantics is the same of current proposal.

In addition to that, on https://2025.stateofjs.com/en-US/features/#js_new_features  it’s the most popular feature that is not in browsers yet other than decorators, and there are some articles and podcasts about it, like https://byteiota.com/typescript-59-import-defer-lazy-imports/ (it talks mostly about TS import defer) and https://youtu.be/_x5f35JVKt8?t=338.

Is there any sign of developer interest we can point to here?

> Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, ChromeOS, Android, and Android WebView)?

> No

I'd expect this to be shipping everywhere -- should this say "yes"?

Sorry, I overlooked this. It’s yes.

[Dan Clark]

Thank you! LGTM1

[Mike Taylor]

LGTM2

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[Philip Jägenstedt]

LGTM3

Looks like the tests are at https://wpt.fyi/results/third_party/test262/test/language?label=master&label=experimental&aligned&q=import-defer and still failing because the flag hasn't been flipped yet. If there are test failures after flipping the flag, can you look into getting them to 100%? (Notably, there's one test failing in Safari, will we pass it?)

To view this discussion visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/fb2bb9ed-4a64-455e-be6d-de19626a80c4%40chromium.org.

[Nicolò Ribaudo]

Those tests are run by V8 as they are in the test262 suite, and there are no failures: they would show up as some entries containing "import-defer" at https://source.chromium.org/chromium/chromium/src/+/main:v8/test/test262/test262.status

That specific Safari failure was a test that we added a few weeks ago while fixing the same bug in V8: https://chromium-review.googlesource.com/c/v8/v8/+/8272408

[Caio Lima]

Thank you very much for reviewing this request.

On Sep 16, 2026, at 6:08 AM, Philip Jägenstedt <foo...@chromium.org> wrote:

LGTM3

Looks like the tests are at https://wpt.fyi/results/third_party/test262/test/language?label=master&label=experimental&aligned&q=import-defer and still failing because the flag hasn't been flipped yet. If there are test failures after flipping the flag, can you look into getting them to 100%? (Notably, there's one test failing in Safari, will we pass it?)

Sure. AFAICT, we are passing all Test262 on current implementation, but if I see any failure, I’ll fix it.

[Philip Jägenstedt]

Den ons 16 sep. 2026 13:06Caio Lima <caio...@igalia.com> skrev:

Thank you very much for reviewing this request.

On Sep 16, 2026, at 6:08 AM, Philip Jägenstedt <foo...@chromium.org> wrote:

LGTM3

Looks like the tests are at https://wpt.fyi/results/third_party/test262/test/language?label=master&label=experimental&aligned&q=import-defer and still failing because the flag hasn't been flipped yet. If there are test failures after flipping the flag, can you look into getting them to 100%? (Notably, there's one test failing in Safari, will we pass it?)

Sure. AFAICT, we are passing all Test262 on current implementation, but if I see any failure, I’ll fix it.

Super, thank you and happy shipping!

[Caio Lima]

BTW, I uploaded the CL[1] to unflag it, but both Reviewers (Olivier Flückiger and Igor Sheludko) that helped me through the development of the feature are OOO, do you know who else could I ask for review instead?

[1] - https://chromium-review.googlesource.com/c/v8/v8/+/8415348