feat: skip `__toESM` helper when only named imports are used from CJS modules by Copilot · Pull Request #6850 · rolldown/rolldown

Copilot · 2025-11-05T04:54:43Z

Original prompt

This section details on the original issue you should resolve

<issue_title>[Feature Request]: skip __toESM when the default export is not used</issue_title>
<issue_description>### What problem does this feature solve?

__toESM helper brings in many helper functions and makes the bundle size bigger.

__toESM helper exists to add the default export in some condition. While the condition is not known until runtime, we can know whether the default import is used or not. When the default import is not used, we can skip the __toESM helper.

For this input:
// index.js
import { foo } from './foo.cjs'
console.log(foo)
// foo.cjs
exports.foo = 'foo'
Rolldown currently outputs:
// omit helper implementations
//#region foo.cjs
var require_foo = __commonJS({ "foo.cjs"(exports) {
	exports.foo = "foo";
} });

//#endregion
//#region index.js
var import_foo = __toESM(require_foo());
console.log(import_foo.foo);

//#endregion
But since we know that the default import is not used, we can improve this to:
// omit helper implementations
//#region foo.cjs
var require_foo = __commonJS({ "foo.cjs"(exports) {
	exports.foo = "foo";
} });

//#endregion
//#region index.ts
var import_foo = require_foo();
console.log(import_foo.foo);

//#endregion
What does the proposed API look like?

Not an API change</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes [Feature Request]: skip __toESM when the default export is not used #6849

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graphite-app · 2025-11-05T04:54:49Z

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Add optimization to skip __toESM helper when only named imports are used from CJS modules. This reduces bundle size by avoiding unnecessary helper code for named-only imports. Co-authored-by: sapphi-red <[email protected]>

Also update reference_needed_symbols to conditionally reference __toESM only for namespace or default imports, not for named-only imports. This eliminates the helper from the runtime completely when it's not needed. Co-authored-by: sapphi-red <[email protected]>

Co-authored-by: sapphi-red <[email protected]>

Move the logic for checking if an import needs interop to a shared utility function to avoid code duplication between module_finalizers and reference_needed_symbols. Co-authored-by: sapphi-red <[email protected]>

Co-authored-by: sapphi-red <[email protected]>

github-actions · 2025-11-05T06:14:24Z

Benchmarks Rust

target: main(35fafd0)
pr: copilot/skip-toesm-when-default-unstated(39464c0)

group                                                        pr                                     target
-----                                                        --                                     ------
bundle/bundle@multi-duplicated-top-level-symbol              1.02     68.0±1.68ms        ? ?/sec    1.00     66.6±1.59ms        ? ?/sec
bundle/bundle@multi-duplicated-top-level-symbol-sourcemap    1.01     75.0±1.94ms        ? ?/sec    1.00     74.2±3.09ms        ? ?/sec
bundle/bundle@rome_ts                                        1.00    111.5±3.09ms        ? ?/sec    1.00    111.8±2.77ms        ? ?/sec
bundle/bundle@rome_ts-sourcemap                              1.01    124.8±2.62ms        ? ?/sec    1.00    123.8±1.70ms        ? ?/sec
bundle/bundle@threejs                                        1.00     40.5±0.89ms        ? ?/sec    1.00     40.5±2.47ms        ? ?/sec
bundle/bundle@threejs-sourcemap                              1.01     44.2±1.14ms        ? ?/sec    1.00     43.6±0.79ms        ? ?/sec
bundle/bundle@threejs10x                                     1.00    400.9±4.69ms        ? ?/sec    1.00    400.2±8.13ms        ? ?/sec
bundle/bundle@threejs10x-sourcemap                           1.00    460.6±4.04ms        ? ?/sec    1.00    459.0±3.32ms        ? ?/sec
scan/scan@rome_ts                                            1.00     87.4±1.62ms        ? ?/sec    1.01     88.7±1.79ms        ? ?/sec
scan/scan@threejs                                            1.00     29.2±0.43ms        ? ?/sec    1.02     29.9±2.11ms        ? ?/sec
scan/scan@threejs10x                                         1.01    307.0±6.15ms        ? ?/sec    1.00    303.2±4.12ms        ? ?/sec

crates/rolldown/src/module_finalizers/mod.rs

crates/rolldown/tests/rolldown/topics/bundler_esm_cjs_tests/51/rolldown.config.mjs

- Inline import_record_needs_interop call directly in if condition - Remove manually added rolldown.config.mjs files from test directories Co-authored-by: sapphi-red <[email protected]>

hyf0 · 2025-11-05T17:41:49Z

Merge activity

Nov 5, 5:41 PM UTC: The merge label 'graphite: merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Nov 5, 5:44 PM UTC: The merge label 'graphite: merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.

@sapphi-red

## [1.0.0-beta.48] - 2025-11-10 :boom: Breaking Changes - `this.emitFile` now respects `chunkFileNames` for chunk type ```js // rolldown.config.js export default { output: { chunkFileNames: 'chunks/[name]-[hash].js' } } // In plugin this.emitFile({ type: 'chunk', id: './my-module.js' }); // Before: Output might not follow chunkFileNames pattern // After: Output follows 'chunks/[name]-[hash].js' pattern ``` - Deprecated top-level options removed - `define` → `transform.define` - `inject` → `transform.inject` - `dropLabels` → `transform.dropLabels` - `keepNames` → `output.keepNames` - `profilerNames` → `output.generatedCode.profilerNames` - Stable plugins moved from experimental ```js // Before import { replacePlugin, esmExternalRequirePlugin } from 'rolldown/experimental'; // After import { replacePlugin, esmExternalRequirePlugin } from 'rolldown/plugins'; ``` - `RolldownBuild#scan` is removed, now only available from `rolldown/experimental` ```js // Before: scan was a method on RolldownBuild const build = await rolldown(config); await build.scan(); // After: import scan from rolldown/experimental import { scan } from 'rolldown/experimental'; await scan(config); ``` ### 💥 BREAKING CHANGES - `this.emitFile` does not respect `chunkFileNames` (#6868) by @Copilot - remove deprecated top-level `dropLabels` option (#6915) by @sapphi-red - remove deprecated top-level `keepNames` option (#6914) by @sapphi-red - remove deprecated top-level `profilerNames` option (#6913) by @sapphi-red - remove deprecated top-level `define` and `inject` options (#6912) by @sapphi-red - move stable plugins from experimental to `rolldown/plugins` (#6303) by @shulaoda - node: remove experimental `RolldownBuild#scan`, only expose it from `rolldown/experimental` (#6889) by @hyf0 ### 🚀 Features - add side-effect detection for global constructors with primitive arguments (#6898) by @IWANABETHATGUY - rust: use `BundleMode` to handle incremental build exhaustively (#6894) by @hyf0 - detect side-effect-free global function calls (#6897) by @IWANABETHATGUY - expose `parseSync` / `parseAsync` function (#6866) by @sapphi-red - skip `__toESM` helper when only named imports are used from CJS modules (#6850) by @Copilot - rolldown_binding: expose `htmlInlineProxyPlugin` (#6856) by @shulaoda - rolldown_plugin_html_inline_proxy: align `load` hook logic (#6855) by @shulaoda - rolldown_plugin_html_inline_proxy: align `resolveId` hook logic (#6854) by @shulaoda - rolldown_plugin_html_inline_proxy: initialize (#6853) by @shulaoda ### 🐛 Bug Fixes - cli: support nested options in CLI properly (#6911) by @sapphi-red - debug: ensure injecting `hook_resolve_id_trigger` correctly (#6908) by @hyf0 - use chunk-specific exports for entry module export detection (#6904) by @IWANABETHATGUY - debug: ensure build get injected and add tests (#6896) by @hyf0 - error: return friendly error for bundler already closed scenario (#6878) by @hyf0 - improve dynamic entry processing with iterative approach (#6869) by @IWANABETHATGUY - handle tsconfig option resolve error (#6871) by @sapphi-red - handle error when creating output chunk directories (#6870) by @sapphi-red - node: `NormalizedOutputOptionsImpl` and `NormalizedInputOptionsImpl` enumerable (#6861) by @hyf0 - node: keys of `RolldownOutput` should be enumerable (#6852) by @Copilot ### 🚜 Refactor - rust: rename `BundleContext` to `BundleHandle` (#6893) by @hyf0 - rust: rename `build_span` to `bundle_span` (#6892) by @hyf0 - rust: introduce `PluginDriverFactory` to manage creation of `PluginDriver` (#6891) by @hyf0 - crates/rolldown_binding: remove useless `BindingBundlerImpl` (#6888) by @hyf0 - crates/rolldown_binding: rename `Bundler` to `ClassicBundler` and clarify the purpose (#6887) by @hyf0 - rust: rename `BuildFactory/Build` to `BundleFactory/Bundle` (#6886) by @hyf0 - rust: tweak incremental build related methods of `Bundler` (#6884) by @hyf0 - rust: manage build via `BuildFactory` for `Bundler` (#6883) by @hyf0 - node: implement a new `Bundler` that satisfy the usage of `RolldownBuild` (#6877) by @hyf0 - node: remove useless `nonEnumerable` decorator (#6862) by @hyf0 ### 📚 Documentation - add documentation for native replace plugin (#6315) by @shulaoda - in-depth/directives: expand description of how directives are handled (#6882) by @sapphi-red - in-depth/bundling-cjs: clarify the condition of `default` export interop (#6875) by @sapphi-red - add troubleshooting section for `this` in exported functions (#6865) by @sapphi-red - update prebuilt binaries list based on Node 24 platform support tier (#6864) by @sapphi-red - remove unsupported [ext] placeholder from entryFileNames and chunkFileNames (#6859) by @Copilot ### ⚡ Performance - rolldown: improve sourcemap chain processing (#6858) by @Brooooooklyn ### 🧪 Testing - add test case for issue #6881 with scientific notation (#6906) by @IWANABETHATGUY ### ⚙️ Miscellaneous Tasks - deps: update npm packages (#6918) by @renovate[bot] - deps: update dependency rolldown-plugin-dts to v0.17.5 (#6917) by @renovate[bot] - deps: lock file maintenance (#6907) by @renovate[bot] - deps: update rust crates (#6905) by @renovate[bot] - deps: update npm packages (#6903) by @renovate[bot] - deps: update github-actions (#6902) by @renovate[bot] - deps: update `oxc_resolver` and `oxc_resolver_napi` (#6901) by @shulaoda - deps: update dependency rolldown-plugin-dts to v0.17.4 (#6895) by @renovate[bot] - deps: update dependency tsdown to v0.16.1 (#6885) by @renovate[bot] - deps: upgrade napi to remove linker args that skip missing symbols (#6867) by @Boshen Co-authored-by: shulaoda <[email protected]>

…mports are used from a CJS module (#7094) `canonical name not found for "__toESM"` error when only named imports are used from a CJS module and SafelyMergeCjsNs optimization is applied. refs #7006, #6850

Initial plan

86aeaa8

Copilot AI assigned Copilot and sapphi-red Nov 5, 2025

Copilot started work on behalf of sapphi-red November 5, 2025 04:55 View session

Copilot AI and others added 5 commits November 5, 2025 05:08

feat: skip __toESM when default export is not used

f928b5c

Add optimization to skip __toESM helper when only named imports are used from CJS modules. This reduces bundle size by avoiding unnecessary helper code for named-only imports. Co-authored-by: sapphi-red <[email protected]>

fix: remove trailing whitespace in module_finalizers

88398f8

Co-authored-by: sapphi-red <[email protected]>

refactor: extract import_record_needs_interop to shared utility

58b4f4e

Move the logic for checking if an import needs interop to a shared utility function to avoid code duplication between module_finalizers and reference_needed_symbols. Co-authored-by: sapphi-red <[email protected]>

fix: apply rustfmt formatting

f084739

Co-authored-by: sapphi-red <[email protected]>

Copilot AI changed the title ~~[WIP] Add feature to skip __toESM helper when default export is unused~~ feat: skip __toESM helper when only named imports are used from CJS modules Nov 5, 2025

Copilot AI requested a review from sapphi-red November 5, 2025 05:40

Copilot finished work on behalf of sapphi-red November 5, 2025 05:40

sapphi-red reviewed Nov 5, 2025

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crates/rolldown/src/module_finalizers/mod.rs Outdated Show resolved Hide resolved

crates/rolldown/src/module_finalizers/mod.rs Show resolved Hide resolved

crates/rolldown/tests/rolldown/topics/bundler_esm_cjs_tests/51/rolldown.config.mjs Outdated Show resolved Hide resolved

Copilot started work on behalf of sapphi-red November 5, 2025 06:24 View session

refactor: simplify interop check and remove test config files

1742b87

- Inline import_record_needs_interop call directly in if condition - Remove manually added rolldown.config.mjs files from test directories Co-authored-by: sapphi-red <[email protected]>

Copilot AI requested a review from sapphi-red November 5, 2025 06:34

Copilot finished work on behalf of sapphi-red November 5, 2025 06:34

sapphi-red approved these changes Nov 5, 2025

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sapphi-red marked this pull request as ready for review November 5, 2025 08:05

sapphi-red requested a review from IWANABETHATGUY November 5, 2025 08:05

sapphi-red assigned IWANABETHATGUY and unassigned sapphi-red Nov 5, 2025

IWANABETHATGUY approved these changes Nov 5, 2025

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IWANABETHATGUY enabled auto-merge (squash) November 5, 2025 14:40

sapphi-red disabled auto-merge November 5, 2025 17:24

Merge branch 'main' into copilot/skip-toesm-when-default-unstated

39464c0

sapphi-red enabled auto-merge (squash) November 5, 2025 17:25

hyf0 added the graphite: merge label Nov 5, 2025

sapphi-red merged commit 64dfc87 into main Nov 5, 2025
32 checks passed

sapphi-red deleted the copilot/skip-toesm-when-default-unstated branch November 5, 2025 17:42

github-actions bot mentioned this pull request Nov 10, 2025

release: v1.0.0-beta.48 #6920

Merged

Copilot AI mentioned this pull request Nov 12, 2025

feat: skip __toCommonJS helper when module.exports export exists #6969

Closed

sapphi-red mentioned this pull request Nov 14, 2025

[Bug]: CJS default export is missing when the same module is imported with named imports and default imports #7004

Closed

This was referenced Nov 14, 2025

Fix: CJS default export missing when module imported with both named and default imports #7005

Closed

fix: missing CJS default export when SafelyMergeCjsNs optimization is enabled #7006

Merged

[WIP] Fix missing CJS default export in module imports #7007

Closed

sapphi-red mentioned this pull request Nov 17, 2025

fix: canonical name not found for "__toESM" error when only named imports are used from a CJS module #7094

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feat: skip `__toESM` helper when only named imports are used from CJS modules#6850

feat: skip `__toESM` helper when only named imports are used from CJS modules#6850
sapphi-red merged 8 commits intomainfrom
copilot/skip-toesm-when-default-unstated

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