Intent to prototype and ship: Source Phase Imports

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Daniel Minor

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Jun 1, 2026, 11:29:40 AM (5 days ago) Jun 1

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As of Firefox 153, I intend to turn Source Phase Imports on by default. It has been developed behind the javascript.options.experimental.source_phase_imports preference.

Summary:
Source Phase Imports allows for modules to be imported in a newly defined `source` phase that skips module linking and evaluation. The proposal itself does not specify any module types that support this new source phase, but the underlying capability is used for wasm/esm integration (https://webassembly.github.io/esm-integration/js-api/index.html) and ESM phase imports (https://github.com/tc39/proposal-esm-phase-imports). Although not directly useful on it's own, shipping it now will allow us to simplify our build and preference configuration, reducing complexity for implementing these, and other, module related proposals.

Bug to turn on by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=2043242

Specification:
https://github.com/tc39/proposal-source-phase-imports

Standards Body:
TC39

Platform Coverage:
All

Preference:
javascript.options.experimental.source_phase_imports

DevTools Bug:
N/A

Extensions Bug:
N/A

Use Counter:
N/A

Standards-Positions Discussion:
N/A

Other Browsers:

In progress, tracked by: https://issues.chromium.org/issues/42204365
Status unclear, may be tracked by this bug for source phase imports of wasm modules: https://bugs.webkit.org/show_bug.cgi?id=274908