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Inside the body of a `with` statement, a name can resolve to a property
of the `with` object. That property shadows a `const` of the same name
from an enclosing scope. The parser ignored this in two places.
Const inlining replaced a read in the body with the value of the const.
`function f(o) { const x = 1; with (o) { return x; } }` returned 1 for
`f({ x: 2 })`. Node returns 2. `handle_identifier` now skips the
substitution when the identifier was resolved through a `with` scope.
An assignment in the body was a parse error ("Cannot assign to "x"
because it is a constant"), although the target can be the property.
This is the fourth snippet of #13992. The error is now skipped inside a
`with` body. The bundler keeps it, because it can print `const` as `let`
or `var`.
The pass that removes inlined const declarations runs once per switch
case, before later cases are visited. A later case can now hold a read
that is not inlined, so that pass leaves switch case lists alone.
The runtime transpiler has inlining on, so its cached output changes.
`EXPECTED_VERSION` in `RuntimeTranspilerCache.rs` moves to 31.
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Updated 4:57 PM PT - Sep 12th, 2026
✅ @robobun, your commit a48da58b2398737b02b60c76445816c513777c5c passed in 🧪 To try this PR locally: bunx bun-pr 42524That installs a local version of the PR into your bun-42524 --bun |
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Status
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WalkthroughThe parser preserves dynamic ChangesWith-scope const resolution
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to A switch with cross-case const usage inside a with scope can emit incorrect code. The fix is localized and should be made before merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Comment |
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…ts `const` as `var` `select_local_kind` prints a top-level `const` as `var` when the bundler runs, and also when a lowered top-level `using` wraps the module in a try block. In both cases nothing stops an assignment to the name at run time. So the const assignment error inside a `with` body now stays in both cases, not only when bundling.
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Inline comments:
In `@src/js_parser/visit/mod.rs`:
- Around line 1810-1813: Keep the single-use const substitution guard in
visit_stmts disabled for StmtsKind::SwitchStmt, so declarations shared across
switch cases are not removed before later case reads—including dynamic with
lookups—are visited.
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src/js_parser/p.rssrc/js_parser/visit/mod.rssrc/js_parser/visit/visit_expr.rs
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Thanks — commit e1c6eb9 addresses the earlier note: the const-assignment gate now mirrors select_local_kind's exact condition (p.options.bundle || p.will_wrap_module_in_try_catch_for_using), and the new transpiler.test.js case pins both the target: "browser" throw and the target: "bun" accept. I re-read the updated diff and didn't find further issues; a human look is still worthwhile given this changes transpiler output semantics for every sloppy-mode file.
What was reviewed
visit_expr.rs: new gate matchesselect_local_kindat p.rs:6641;will_wrap_module_in_try_catch_for_usingis set at parse_entry.rs:959 before the visit pass, so it's populated when read here.visit/mod.rs:StmtsKind::SwitchStmtskip — the removal pass runs per case body; the guard is conservative and covered by thelaterCaseruntime test andtranspiler.test.jsoutput check.RuntimeTranspilerCache.rs: version bump to 31 with a changelog line, consistent with the file's convention.- Tests:
test.concurrent+tempDir/bunExe/bunEnv, stdout/stderr asserted before exitCode, pipes drained viaPromise.all.
Extended reasoning...
Overview
Three small parser edits stop const-inlining and the const-assignment diagnostic from misfiring inside with bodies, plus a StmtsKind::SwitchStmt guard on the inlined-const removal pass and a transpiler-cache version bump. Roughly 10 lines of source change backed by ~200 lines of tests across bundler_minify.test.ts, runtime-transpiler.test.ts, and transpiler.test.js.
Security risks
None. This is a correctness fix in the JS transpiler's constant-folding pass; no I/O, auth, or untrusted-input parsing surface changes.
Level of scrutiny
High — the parser/visit pass runs on every transpiled file, and any change to when a const is inlined or when the const-assignment error fires is observable across the runtime, Bun.Transpiler, and the bundler. Since the last review, the author added the will_wrap_module_in_try_catch_for_using arm and a test pinning it; the gate now matches select_local_kind's lowering condition byte-for-byte, which was the concern. A human should still confirm the intentional behavior differences (bundler keeps the error; runtime/transpiler accept it) and the switch-case removal skip's minor minification cost noted in the PR description.
Other factors
Test coverage is thorough for the variant matrix REVIEW.md asks for: reads (plain, closure, nested, typeof, top-level, in-body, switch fall-through), assignments (=, +=, ++, destructuring, for-in, closure, non-literal, missing-property TypeError, the #13992 snippet), plus bundler and lowered-using exceptions. Tests follow harness conventions (test.concurrent, tempDir, Promise.all pipe drain, assert output before exitCode). An unresolved coderabbitai inline comment remains on visit/mod.rs:1813; I can't see its content, so leaving that for human triage.
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Problem
function f(o) { const x = 1; with (o) { return x; } }returns1forf({ x: 2 })underbun file.cjs. Node returns2:o.xshadows the const.P::handle_identifier(src/js_parser/p.rs:2182) replaces every read of a symbol inconst_valuesand ignores thewithscope.const obj = { obj: 2 }; with (obj) { obj = 10; }does not load:error: This assignment will throw because "obj" is a constant. Node assigns to the property. This is the fourth snippet of Unexpected behaviour in Bun's static analysis. #13992. The check is ine_identifier(src/js_parser/visit/visit_expr.rs:200).Fix
handle_identifierskips the substitution whenident.must_keep_due_to_with_stmt()is set.EXPECTED_VERSIONmoves to 31: the runtime transpiler's cached output changes.e_identifierskips the const assignment error inside awithbody. The error stays whereselect_local_kindcan printconstasletorvar: in the bundler, and below a lowered top-levelusing.constdeclarations (src/js_parser/visit/mod.rs:1810) skips switch case lists. It runs per case, before later cases are visited. A kept read in a later case would lose its declaration.runtime-transpiler.test.ts,transpiler.test.js,bundler_minify.test.ts. Five fail on bun 1.4.3. Also rantest/bundler/transpiler/,esbuild/default,esbuild/dce.Background
constinconst_valuesand replaces later reads. A declaration with no reads left is removed.with (o) { ... }putsoin the scope chain of its body. A bare name there reads or writeso.namewhenohas that property.find_symbolsetsis_inside_with_scopewhen the lookup passes awithscope before the declaration.e_identifiercopies it to the identifier. User defines already skip such identifiers (visit_expr.rs:257).switchshare one scope. Each case body is visited as its own statement list.Notes
Origin. The read half comes from a read of the code, not from a user report. The assignment half is the fourth snippet of #13992 ("the const reassignment error shouldn't be triggering when not in strict mode"). The other three snippets of #13992 are not changed. esbuild has the same gap in
handleIdentifier(checked againstevanw/esbuildmain), so the read half is a deviation from esbuild toward Node on purpose.What still inlines. A const declared in the body's own block (
with (o) { const x = 1; return x; }) is found before the lookup reaches thewithscope, so it is still inlined. A read outside the body is still inlined. Thewith (expr)object expression is outside the body and is still inlined.The assignment check and
select_local_kind. When bundling,select_local_kindprints a top-levelconstasvar, and with--minify-syntaxa nestedconstaslet. Without the bundler it also prints a top-levelconstasvarwhen a lowered top-levelusingwraps the module in atryblock (will_wrap_module_in_try_catch_for_using, for example--target=browser). That is only safe when nothing assigns to the name. So in both cases the const assignment error stays inside awithbody. Otherwisebun run,bun build --no-bundle, andBun.Transpileraccept the assignment. Ifodoes not have the property, the assignment reaches the const and JSC throws aTypeErrorat run time, as in Node. Without thehandle_identifierchange, the accepted assignment would print as1 = 5.The switch case guard. On main every read of an inlined const that is visited after its declaration is replaced, so the early removal in a switch case was harmless. With this PR a read inside a
withbody stays. Forswitch (k) { case 1: const x = "const"; case 2: with (o) { return x; } }the declaration was removed when case 1 ended, and the read in case 2 threwReferenceError: x is not defined. The guard costs one dead statement in minified output:switch (k) { case 1: const x = 1; return x; }printscase 1: const x = 1; return 1;. Braced case bodies have their own scope and are not affected. No existing test depends on the removal. esbuild now disables const inlining in switch cases entirely. #41848 runs the removal after every case is visited, which makes this guard unnecessary. #40791 and #30936 touch the same area. Each of them conflicts with main today, so this PR does not build on them.TypeScript is not changed.
with (o) { return E.A; }in a.ctsfile still prints1 /* A */. TypeScript rejectswith(TS2410, and TS1101 because TS 6 files are strict), and tsc 6.0.2 itself inlinesconst enummembers inside awithbody (return 7 /* C.X */).Adjacent sites, not changed here.
evalcheck to the sameifline inhandle_identifier. The conflict is textual only.new Array(1, 2)) insidewithare handled in a separate change insrc/ast/known_global.rs.function f() { const x = 1; return delete x; }printsdelete 1. This does not depend onwith.EXPECTED_VERSIONto 31. The one that lands second moves it again.Test matrix. The run-time fixtures are sloppy
.cjsfiles. Each expected value is what Node v26.3.0 prints. Reads: plain, in a closure, nestedwith,typeof, top-level const, const in the body's own block, switch fall-through. Assignments:=,+=,++, array destructuring,for-intarget, in a closure, a const that is not a literal, the missing-propertyTypeError, and the #13992 snippet.minify/ConstAssignInsideWithIsStillABundleErrorpins the bundler exception. It passes with and without this change.transpiler.test.jspins the loweredusingexception (target: "browser"throws,target: "bun"accepts).[human-review] gate passed · iteration 0 · 7 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 0
evidence per changed file