ESM Module Loading: Cyclic Module Records / multiple top-level awaits in different modules

[mb21]

Description

While officially supporting ESM modules, even with top-level await, WebKit doesn't actually implement the module loader spec. This leads to a race condition, where one or more modules fail to load if both of them have a top-level await.

Specification

TC39: https://tc39.es/ecma262/#cyclic-module-record

web-feature

js-modules

Tests

• A minimal reproducible module, that leads to the race condition when imported twice is this module:

  export let bar = 'foo'
  await 0

• @jakearchibald added this repro in the comments below, which seemingly depending on network conditions, fails in Safari when a long enough timeout is used.

• The language/module-code/top-level-await/fulfillment-order.js test (source) on test262.fyi currently also fails for JavaScriptCore, which at the very least is closely related.

Additional Signals

@shvaikalesh wrote on the stalled WebKit PR:

I don't see any other way except for re-implementing the module loader to adhere to the spec (https://tc39.es/ecma262/#cyclic-module-record), which will take some time.

And @MenloDorian replied with:

This PR requires significant feature-level work to complete. We don't currently have anyone working on it, but we hear the requests and will consider it in our planning.

Motivation

A lot of people using modern frameworks like SvelteKit or Astro (that use native ESM modules) are running into this, and it's often very hard to debug, so they don't realize it's this webkit bug.

[github-actions]

This comment was automatically generated based on the information you provided. Please don't edit it.

Below is additional information about the web feature (from the web-features project) which is referenced in your proposal.
If this doesn't accurately correspond to your proposal, please update your initial comment to include web-features: <feature-id>.
To find feature IDs, use the web-features explorer.

Feature JavaScript modules

• ID: js-modules
• Name: JavaScript modules
• Description: JavaScript modules allow code to be organized into reusable units. Modules use import to load other modules and export to declare what is available to import from other modules. In HTML, modules are loaded with <script type="module">.
• Baseline status: Widely Available
• Docs: <script> type attribute
• Chrome use counter: chromestatus.com
• State of CSS/JS/HTML surveys: State of HTML 2024 (web_components question), State of HTML 2023 (web_components question), State of HTML 2023 (features question)
• More information: See the web-features explorer

[Rich-Harris]

Thank you for opening this @mb21!

It's hard to overstate the significance of this bug. It's not just that modern frameworks like SvelteKit and Astro have to invent convoluted workarounds, it's that web developers cannot reliably use top-level await in their own code if they want it to run in Safari, which effectively means web developers cannot use top-level await at all. This is holding the web back.

[jakearchibald]

https://random-stuff.jakearchibald.com/bug-repros/top-level-await-webkit/ - here's a reduced version of the reproduction in the WebKit report

[jakearchibald]

• @jakearchibald added this repro in the comments below, which seemingly depending on network conditions, often fails in Safari.

You can use ?timeout=1000 etc on the URL to increase the wait time, which may avoid the race condition.

[dasa]

Here's another TLA test app that demonstrates this WebKit issue: https://maxpatiiuk.github.io/safari-top-level-await-bug/

[ddbeck]

The web-features entry involved here is top-level-await (web features explorer).

[gsnedders]

Tests

There's also https://wpt.fyi/results/html/semantics/scripting-1/the-script-element/module/top-level-await?label=master&label=experimental&aligned, which has a single test — see web-platform-tests/wpt#42944 for why that exists in WPT (as well as in test262).

[wpt-interop]

Thank you for proposing ESM Module Loading: Cyclic Module Records / multiple top-level awaits in different modules for Interop 2026.

We are pleased to let you know that it has been accepted as part of the Web compat focus area.

You’ll be able to track progress on this topic throughout the year via the Interop 2026 dashboard.

Posted on behalf of the Interop team.