Transform code to support top-level await in normal browsers for Vite. Support all modern browsers of Vite's default target without need to set build.target to esnext.

yarn add -D vite-plugin-top-level-await

Put this plugin in your plugin list. At most case you don't need to care the order, but if there're any plugin transforming bundle before it, there's a little chance that this plugin fails to parse code since it does only parse Rollup's output export { ... } export statement.

import topLevelAwait from "vite-plugin-top-level-await";

export default defineConfig({
  plugins: [
    topLevelAwait({
      // The export name of top-level await promise for each chunk module
      promiseExportName: "__tla",
      // The function to generate import names of top-level await promise in each chunk module
      promiseImportName: i => `__tla_${i}`
    })
  ]
});

You can use this plugin for workers (by putting it in config.worker.plugins).

• If the worker format is ES, the plugin works normally.
• If the worker format is IIFE, the plugin first let Vite build your worker as an ES bundle since IIFE doesn't support top-level awaits, and then build the transformed ES bundle to IIFE. Please use IIFE when targeting Firefox.

  const myWorker = import.meta.env.DEV
      // In development mode, `import`s in workers are not transformed, so you
      // must use `{ type: "module" }`.
    ? new Worker(new URL("./my-worker.js", import.meta.url), { type: "module" })
      // In build mode, let Vite and vite-plugin-top-level-await build a single-file
      // bundle of your worker that works on both modern browsers and Firefox.
    : new Worker(new URL("./my-worker.js", import.meta.url), { type: "classic" });

This plugin transforms code from:

import { a } from "./a.js"; // This module uses top-level await
import { b } from "./b.js"; // This module uses top-level await too
import { c } from "./c.js"; // This module does NOT use top-level await

const x = 1;
await b.func();
const { y } = await somePromise;

export { x, y };

To:

import { a, __tla as __tla_0 } from "./a.js"; // This module uses top-level await
import { b, __tla as __tla_1 } from "./b.js"; // This module uses top-level await too
import { c } from "./c.js"; // This module does NOT use top-level await

// Original exported variables
let x, y;

// Await imported TLA promises and execute original top-level statements
let __tla = Promise.all([
  (() => { try { return __tla_0; } catch {} })(),
  (() => { try { return __tla_1; } catch {} })()
]).then(async () => {
  // Transform exported variables to assignments
  x = 1;

  await b.func();

  // Destructing patterns (and function / class declarations as well) are handled correctly
  ({ y } = await somePromise);
});

// Export top-level await promise
export { x, y, __tla };

It could handle correct usage of circular dependencies with the default behavior of ES standard. But when an TLA dependency is being awaited, an accessing to one of its exports will NOT raise an exception. At most time you don't need to care about this. These could be supported by doing more transformations of the whole AST but it will make building a lot slower. Open an issue and tell me your scenario if you really need the exception.