Module sync assert

Problem to solve

Some code must be synchronous. If the module accidentally becomes async (by having a top-level await or the old semantics of WebAssembly ESM integration in the subgraph) the code might break in a way that hard to debug.

Example: Service Worker

import './some-module.js'
addEventListener('fetch', () => {})

Note

If you try to use the native implementation of the ES Module in Service Worker, it will throw a TypeError "Top-level await is disallowed in service workers.".

Let's assume the service worker is bundled via a bundler and transformed into a non-ES module format.

If some-module.js becomes async, then the addEventListener no longer works:

Warning

Event handler of 'fetch' event must be added on the initial evaluation of worker script.

Example: Polyfill

The polyfill code must run synchronously, otherwise, the application might be broken in old browsers.

Example: API stability

Adding TLA is a breaking change, the library author may want to add a test that their library won't accentually become async due to dependency changes.

Solution

Adding a hint to the engine, if the module evaluation is async or contains an async subgraph, the engine should fail early (syntax error), so the developer can fix it early.

Directive

"assert sync"

Adding a new directive to hint the engine.

Other solutions: Linter/bundler level bans

It is possible, but each tool needs to invent its convention to do this. It also does not apply to developers that don't use a bundler/linter.