fix(es/transforms): avoid rewriting unknown relative extensions by kdy1 · Pull Request #11713 · swc-project/swc

kdy1 · 2026-03-20T00:44:13Z

Summary

This PR fixes jsc.rewriteRelativeImportExtensions rewriting unknown relative extensions to .js.

Previously, specifiers like ./index.legacy were rewritten to ./index.js because unknown extensions fell through to a default rewrite. This diverged from TypeScript behavior.

This change makes rewriting extension-aware and only rewrites supported extensions:

ts | js -> js
tsx | jsx -> jsx
mts | mjs -> mjs
cts | cjs -> cjs
json -> json
unknown extensions -> no rewrite

Also adds fixture coverage for issue #11644 to assert:

import "./index.legacy" stays unchanged
import "./index.legacy.ts" rewrites to import "./index.legacy.js"

Closes #11644.

Testing

git submodule update --init --recursive
cargo test -p swc --test projects ts_id_tests__typescript__rewrite_relative_import_specifier__issue_11644__input -- --exact --ignored --nocapture
cargo test -p swc --test projects rewrite_relative_import_specifier -- --ignored
cargo fmt --all
cargo clippy --all --all-targets -- -D warnings
cargo test -p swc_ecma_transforms_module

cargo test -p swc was also run; unrelated tests/source_map cases failed in this environment because Node module sourcemap-validator is missing.

changeset-bot · 2026-03-20T00:44:19Z

🦋 Changeset detected

Latest commit: 15f192c

The changes in this PR will be included in the next version bump.

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github-actions · 2026-03-20T00:53:28Z

Binary Sizes

File	Size
`swc.linux-x64-gnu.node`	28M (29065032 bytes)

Commit: 5f276f7

codspeed-hq · 2026-03-20T01:01:09Z

Merging this PR will not alter performance

✅ 219 untouched benchmarks

_{Comparing kdy1/fix-11644-rewrite-relative-import-extensions (15f192c) with main (9b3abe0)}

claude · 2026-03-20T01:35:44Z

PR Review: fix(es/transforms): avoid rewriting unknown relative extensions

Summary

Clean, well-scoped fix. The core change (2 lines in import_rewriter_typescript.rs) correctly aligns the static extension rewriting with TypeScript's behavior by only rewriting known extensions and leaving unknown ones untouched.

Code Quality

The fix is correct. The old _ => "js" default was too aggressive — it rewrote specifiers like ./index.legacy to ./index.js, which diverges from TypeScript. Returning None for unknown extensions means no rewrite happens, which is the right behavior.
Explicit "js" | "ts" => "js" arm is good. Previously "js" fell through to the _ => "js" default (a no-op in practice), but making it explicit is clearer and necessary now that _ returns None. It also mirrors the TypeScript source structure referenced in the comments.
Runtime helper is already correct. The regex in _ts_rewrite_relative_import_extension.js only matches known TS extensions, so the runtime path for dynamic require() already handled this correctly. This PR brings the compile-time static path into alignment.

Test Coverage

Good fixture test covering the exact reported issue: ./index.legacy stays unchanged, ./index.legacy.ts rewrites to ./index.legacy.js.
The .swcrc config is appropriate (typescript parser, nodenext module, rewriteRelativeImportExtensions: true).

Minor Observations

No-op rewrite for .js extension: When the specifier already has a .js extension, get_output_extension now returns Some("./foo.js") (same path), which causes callers to set raw = None unnecessarily. This is harmless but technically a no-op write. Not worth changing — just noting it.
Could consider testing more unknown extensions: The fixture covers .legacy. It might be worth also having a case for extensionless relative imports (e.g., ./utils) to ensure those aren't touched either — though path.extension() returning None already handles that via the ? operator on line 95. Not a blocker.

Verdict

Looks good to merge. The fix is minimal, correct, well-tested, and aligned with both the TypeScript reference implementation and the existing runtime helper.

claude · 2026-03-20T01:35:54Z

PR Review

Overall: Clean, well-scoped fix. The change correctly aligns SWC's rewriteRelativeImportExtensions behavior with TypeScript's semantics.

Code Quality

The fix is minimal and correct. Changing the catch-all _ => "js" to _ => return None is the right approach — unknown extensions should be left untouched rather than blindly rewritten to .js. The addition of "js" | "ts" => "js" as an explicit arm is necessary since "ts" was previously handled by the catch-all.

The match arms now explicitly enumerate every supported mapping, making the behavior self-documenting and easy to audit against the TypeScript reference implementation.

Potential Issues

None identified. The logic correctly mirrors the TypeScript compiler's getOutputExtension behavior at the referenced commit. The path.extension() call returns None for paths without an extension (e.g., ./foo), which already falls through to None correctly — so extensionless specifiers are also unaffected.

Test Coverage

The new fixture covers both key cases from the issue:

./index.legacy (unknown extension) — remains unchanged
./index.legacy.ts (known .ts extension with dots in stem) — correctly rewrites to .index.legacy.js

One minor suggestion: consider also covering require() and export ... from with unknown extensions if not already covered by existing tests, since those code paths (visit_mut_call_expr, visit_mut_named_export, visit_mut_export_all) share the same get_output_extension function. That said, since they all funnel through the same function, the current coverage is likely sufficient.

Performance

No concerns — the change is a constant-time match arm adjustment with no new allocations or branching overhead.

Security

No concerns.

LGTM — this is a clean bug fix with appropriate test coverage.

Copilot

Pull request overview

Fixes jsc.rewriteRelativeImportExtensions so unknown relative extensions are no longer rewritten to .js, aligning behavior with TypeScript (issue #11644).

Changes:

Update TypeScript import rewriter to only rewrite a known set of extensions; unknown extensions now result in no rewrite.
Add a regression fixture for ./index.legacy (unchanged) and ./index.legacy.ts (rewritten to .js).
Add a changeset documenting the patch release.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
crates/swc_ecma_transforms_module/src/rewriter/import_rewriter_typescript.rs	Stop defaulting unknown extensions to `.js`; return `None` to skip rewrite.
crates/swc/tests/typescript/rewrite-relative-import-specifier/issue-11644/input/main.ts	New fixture input reproducing the problematic import specifiers.
crates/swc/tests/typescript/rewrite-relative-import-specifier/issue-11644/output/main.ts	New fixture output asserting correct rewrite/no-rewrite behavior.
crates/swc/tests/typescript/rewrite-relative-import-specifier/issue-11644/input/.swcrc	Fixture config enabling `rewriteRelativeImportExtensions` under `nodenext`.
.changeset/smooth-games-knock.md	Patch changeset for `swc_core` and transform crate.

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Copilot · 2026-03-20T01:35:57Z

    let ext = path.extension()?.to_str()?;
    let ext = match ext {
+        "js" | "ts" => "js",
        "json" => "json",
        "jsx" | "tsx" => "jsx",
        "mjs" | "mts" => "mjs",
        "cjs" | "cts" => "cjs",
-        _ => "js",
+        _ => return None,
    };


If the intent is to match TypeScript behavior closely, consider normalizing ext to ASCII-lowercase before matching. As written, specifiers like ./foo.TS (or any mixed-case extension) will not be rewritten even though they are effectively the same extension on case-insensitive platforms.

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chatgpt-codex-connector · 2026-03-20T01:37:41Z

    let ext = match ext {
+        "js" | "ts" => "js",
        "json" => "json",
        "jsx" | "tsx" => "jsx",
        "mjs" | "mts" => "mjs",
        "cjs" | "cts" => "cjs",
-        _ => "js",
+        _ => return None,


Keep mixed-case TS extensions rewriteable

With jsc.rewriteRelativeImportExtensions enabled, literal specifiers such as import "./foo.TS" or export * from "../bar.MTS" now hit _ => return None, so the emitted JS keeps the .TS/.MTS suffix instead of rewriting to .js/.mjs. That is a regression from the previous fallback behavior, and it also diverges from our dynamic-path helper in crates/swc_ecma_transforms_base/src/helpers/_ts_rewrite_relative_import_extension.js:3, which still matches these extensions case-insensitively via /...ts$/i.

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fix(es/transforms): avoid rewriting unknown relative extensions

e4ea516

kdy1 marked this pull request as ready for review March 20, 2026 01:34

kdy1 requested a review from a team as a code owner March 20, 2026 01:34

Copilot AI review requested due to automatic review settings March 20, 2026 01:34

kdy1 self-assigned this Mar 20, 2026

kodiakhq Bot previously approved these changes Mar 20, 2026

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Create smooth-games-knock.md

15f192c

kdy1 dismissed kodiakhq[bot]’s stale review via 15f192c March 20, 2026 01:34

kdy1 requested a review from a team as a code owner March 20, 2026 01:34

kodiakhq Bot approved these changes Mar 20, 2026

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kdy1 merged commit ed09218 into main Mar 20, 2026
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kdy1 deleted the kdy1/fix-11644-rewrite-relative-import-extensions branch March 20, 2026 01:34

github-actions Bot added this to the Planned milestone Mar 20, 2026

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fix(es/transforms): avoid rewriting unknown relative extensions#11713

fix(es/transforms): avoid rewriting unknown relative extensions#11713
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