Dynamic import().then(m => m.foo) works in dev but fails in prod

[privatenumber]

Describe the bug

Dynamically importing a file and extracting the named export with a chained .then(m => m.foo) doesn't seem to work when the project is built (it works fine in dev mode).

index.js

(async () => {
    const { bar } = await import("./foo.js").then((a) => a.foo);
    console.log(bar);
})();

foo.js

export const foo = {
  bar: 123,
};

Error

index-Bew75usO.js:1 Uncaught (in promise) TypeError: Cannot destructure property 'bar' of '(intermediate value)' as it is undefined.
    at index-Bew75usO.js:1:1733

Built code

(function polyfill() {
  const relList = document.createElement("link").relList;
  if (relList && relList.supports && relList.supports("modulepreload")) {
    return;
  }
  for (const link of document.querySelectorAll('link[rel="modulepreload"]')) {
    processPreload(link);
  }
  new MutationObserver((mutations) => {
    for (const mutation of mutations) {
      if (mutation.type !== "childList") {
        continue;
      }
      for (const node of mutation.addedNodes) {
        if (node.tagName === "LINK" && node.rel === "modulepreload")
          processPreload(node);
      }
    }
  }).observe(document, { childList: true, subtree: true });
  function getFetchOpts(link) {
    const fetchOpts = {};
    if (link.integrity) fetchOpts.integrity = link.integrity;
    if (link.referrerPolicy) fetchOpts.referrerPolicy = link.referrerPolicy;
    if (link.crossOrigin === "use-credentials")
      fetchOpts.credentials = "include";
    else if (link.crossOrigin === "anonymous") fetchOpts.credentials = "omit";
    else fetchOpts.credentials = "same-origin";
    return fetchOpts;
  }
  function processPreload(link) {
    if (link.ep)
      return;
    link.ep = true;
    const fetchOpts = getFetchOpts(link);
    fetch(link.href, fetchOpts);
  }
})();
const scriptRel = "modulepreload";
const assetsURL = function(dep) {
  return "/" + dep;
};
const seen = {};
const __vitePreload = function preload(baseModule, deps, importerUrl) {
  let promise = Promise.resolve();
  if (deps && deps.length > 0) {
    document.getElementsByTagName("link");
    const cspNonceMeta = document.querySelector(
      "meta[property=csp-nonce]"
    );
    const cspNonce = (cspNonceMeta == null ? void 0 : cspNonceMeta.nonce) || (cspNonceMeta == null ? void 0 : cspNonceMeta.getAttribute("nonce"));
    promise = Promise.allSettled(
      deps.map((dep) => {
        dep = assetsURL(dep);
        if (dep in seen) return;
        seen[dep] = true;
        const isCss = dep.endsWith(".css");
        const cssSelector = isCss ? '[rel="stylesheet"]' : "";
        if (document.querySelector(`link[href="${dep}"]${cssSelector}`)) {
          return;
        }
        const link = document.createElement("link");
        link.rel = isCss ? "stylesheet" : scriptRel;
        if (!isCss) {
          link.as = "script";
        }
        link.crossOrigin = "";
        link.href = dep;
        if (cspNonce) {
          link.setAttribute("nonce", cspNonce);
        }
        document.head.appendChild(link);
        if (isCss) {
          return new Promise((res, rej) => {
            link.addEventListener("load", res);
            link.addEventListener(
              "error",
              () => rej(new Error(`Unable to preload CSS for ${dep}`))
            );
          });
        }
      })
    );
  }
  function handlePreloadError(err) {
    const e = new Event("vite:preloadError", {
      cancelable: true
    });
    e.payload = err;
    window.dispatchEvent(e);
    if (!e.defaultPrevented) {
      throw err;
    }
  }
  return promise.then((res) => {
    for (const item of res || []) {
      if (item.status !== "rejected") continue;
      handlePreloadError(item.reason);
    }
    return baseModule().catch(handlePreloadError);
  });
};
(async () => {
  const { bar } = await __vitePreload(async () => {
    const { bar: bar2 } = await Promise.resolve().then(() => foo);
    return { bar: bar2 };
  }, true ? void 0 : void 0).then((m) => m.foo);
  console.log(bar);
})();
const foo = /* @__PURE__ */ Object.freeze(/* @__PURE__ */ Object.defineProperty({
  __proto__: null
}, Symbol.toStringTag, { value: "Module" }));

^ Seems foo.js is completely empty.

This reproduction is super boiled down, but for context, the bug originated from inside a library dependency that's also built with Vite. While avoiding .then(m => m.foo) might be a workaround, that's not easy to do as it's coming from minified dependency code.

Reproduction

https://stackblitz.com/edit/vitejs-vite-jdxhwfbi?file=src%2Findex.js

Steps to reproduce

npm start (which runs vite build && vite preview)

System Info

N/A

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to vuejs/core instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[privatenumber]

To investigate, I experimented with this configuration:

export default {
    build: {
        minify: false,
        rollupOptions: {
            preserveEntrySignatures: true,
            treeshake: false,
            output: {
                preserveModules: true
            },
        }
    },
};

And this shows that this isn't a tree-shaking issue, but rather that the .then() chain is misplaced when __vitePreload() is injected:

Compiled foo.js:

const foo = {
  bar: 123
};
export {
  foo
};

Compiled index.js:

import { __vitePreload } from "../_virtual/preload-helper-DAnU8ViX.js";
(async () => {
  const { bar } = await __vitePreload(async () => {
    const { bar: bar2 } = await import("./foo-Wx1xxurL.js");
    return { bar: bar2 };
  }, true ? [] : void 0).then((a) => a.foo);
  console.log(bar);
})();

The .then((a) => a.foo) should remain right after import(), but instead, it's moved to the end of __vitePreload(). I think this is coming from

vite/packages/vite/src/node/plugins/importAnalysisBuild.ts

Lines 348 to 351 in 998303b

	str().prependLeft(
	expStart,
	`${preloadMethod}(async () => { ${declaration} = await `,
	)

[privatenumber]

I dug deeper into this.

Looks like the __vitePreload() method is wrapping more than just the import() because it needs to annotate which named exports are being accessed for Rollup to tree-shake (added in #14221).

And the case I'm encountering is not handled here:

vite/packages/vite/src/node/plugins/importAnalysisBuild.ts

Lines 258 to 304 in 998303b

	while ((match = dynamicImportTreeshakenRE.exec(source))) {
	/* handle `const {foo} = await import('foo')`
	*
	* match[1]: `const {foo} = await import('foo')`
	* match[2]: `{foo}`
	* import end: `const {foo} = await import('foo')_`
	* ^
	*/
	if (match[1]) {
	dynamicImports[dynamicImportTreeshakenRE.lastIndex] = {
	declaration: `const ${match[2]}`,
	names: match[2]?.trim(),
	}
	continue
	}
	/* handle `(await import('foo')).foo`
	*
	* match[3]: `(await import('foo')).foo`
	* match[4]: `.foo`
	* import end: `(await import('foo'))`
	* ^
	*/
	if (match[3]) {
	let names = /\.([^.?]+)/.exec(match[4])?.[1] || ''
	// avoid `default` keyword error
	if (names === 'default') {
	names = 'default: __vite_default__'
	}
	dynamicImports[
	dynamicImportTreeshakenRE.lastIndex - match[4]?.length - 1
	] = { declaration: `const {${names}}`, names: `{ ${names} }` }
	continue
	}
	/* handle `import('foo').then(({foo})=>{})`
	*
	* match[5]: `.then(({foo})`
	* match[6]: `foo`
	* import end: `import('foo').`
	* ^
	*/
	const names = match[6]?.trim()
	dynamicImports[
	dynamicImportTreeshakenRE.lastIndex - match[5]?.length
	] = { declaration: `const {${names}}`, names: `{ ${names} }` }
	}

I think this regular expression approach is frail and will encounter more edge-cases in the future (I'm thinking many people have already encountered this but just didn't debug/report).

Planning to look into alternative solutions soon.