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fix(es/typescript): Treat const variable references as enum constants #12101

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3fee92c
fix(es/typescript): Treat const variable references as enum constants
baltasarblanco Aug 5, 2026
c093ba8
chore: Add changeset
baltasarblanco Aug 5, 2026
0851e3e
test(es/typescript): Update constEnum2 tsc reference
baltasarblanco Aug 5, 2026
d547a9a
fix(es/typescript): Resolve const initializers against seen enums and…
baltasarblanco Aug 5, 2026
dd1035a
test(es/typescript): Cover enum-member and multi-declarator const fol…
baltasarblanco Aug 5, 2026
f6f9b27
perf(es/typescript): Skip const initializers that cannot be constant
baltasarblanco Aug 5, 2026
8eeebc5
fix(es/typescript): Keep mutable enum reads opaque in const initializers
baltasarblanco Aug 6, 2026
2e8749f
test(es/typescript): Cover const initializers under mutable enums
baltasarblanco Aug 6, 2026
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5 changes: 5 additions & 0 deletions .changeset/fix-ts-enum-const-var-folding.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
swc_core: patch
swc_ecma_transforms_typescript: patch
---
fix(es/typescript): Treat const variable references as enum constants
1 change: 0 additions & 1 deletion crates/swc/tests/tsc-references/constEnum2.1.normal.js
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Expand Up @@ -7,6 +7,5 @@ var CONST = 9000 % 2;
var D = function(D) {
D[D["e"] = 199 * Math.floor(Math.random() * 1000)] = "e";
D[D["f"] = 10 - 100 * Math.floor(Math.random() % 8)] = "f";
D[D["g"] = CONST] = "g";
return D;
}(D || {});
2 changes: 1 addition & 1 deletion crates/swc/tests/tsc-references/constEnum2.2.minified.js
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@@ -1,2 +1,2 @@
//// [constEnum2.ts]
var D, D1 = ((D = D1 || {})[D.e = 199 * Math.floor(1000 * Math.random())] = "e", D[D.f = 10 - 100 * Math.floor(Math.random() % 8)] = "f", D[D.g = 0] = "g", D);
var D, D1 = ((D = D1 || {})[D.e = 199 * Math.floor(1000 * Math.random())] = "e", D[D.f = 10 - 100 * Math.floor(Math.random() % 8)] = "f", D);
48 changes: 46 additions & 2 deletions crates/swc_ecma_transforms_typescript/src/semantic.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@ use swc_ecma_visit::{noop_visit_type, Visit, VisitWith};
use crate::{
retain::{should_retain_decl, IsConcrete},
shared::{enum_member_name, get_module_ident},
ts_enum::{EnumValueComputer, TsEnumRecord, TsEnumRecordKey, TsEnumRecordValue},
ts_enum::{EnumValueComputer, EvalCtx, TsEnumRecord, TsEnumRecordKey, TsEnumRecordValue},
};

#[derive(Debug, Default)]
Expand All @@ -19,6 +19,7 @@ pub(crate) struct SemanticInfo {
pub enum_record: TsEnumRecord,
pub const_enum: FxHashSet<Id>,
pub namespace_import_equals_usage: FxHashSet<Span>,
pub const_vars: FxHashMap<Id, TsEnumRecordValue>,
}

impl SemanticInfo {
Expand Down Expand Up @@ -48,6 +49,7 @@ pub(crate) fn analyze_program(
unresolved_mark: Mark,
seed_usage: FxHashSet<Id>,
flow_syntax: bool,
ts_enum_is_mutable: bool,
) -> SemanticInfo {
let mut analyzer = SemanticAnalyzer {
unresolved_ctxt: SyntaxContext::empty().apply_mark(unresolved_mark),
Expand All @@ -60,6 +62,7 @@ pub(crate) fn analyze_program(
namespace_id: None,
skip_transform_info: false,
flow_syntax,
ts_enum_is_mutable,
};

program.visit_with(&mut analyzer);
Expand All @@ -75,6 +78,7 @@ struct SemanticAnalyzer {
namespace_id: Option<Id>,
skip_transform_info: bool,
flow_syntax: bool,
ts_enum_is_mutable: bool,
}

#[derive(Default)]
Expand Down Expand Up @@ -257,6 +261,7 @@ impl SemanticAnalyzer {
enum_id: &Id,
default_init: &TsEnumRecordValue,
record: &TsEnumRecord,
const_vars: &FxHashMap<Id, TsEnumRecordValue>,
unresolved_ctxt: SyntaxContext,
flow_syntax: bool,
) -> TsEnumRecordValue {
Expand All @@ -267,8 +272,10 @@ impl SemanticAnalyzer {
enum_id,
unresolved_ctxt,
record,
const_vars,
const_enum_only: None,
}
.compute(expr)
.compute(expr, EvalCtx::MEMBER)
})
.filter(TsEnumRecordValue::has_value)
.unwrap_or_else(|| {
Expand Down Expand Up @@ -536,6 +543,40 @@ impl Visit for SemanticAnalyzer {
}
}

fn visit_var_decl(&mut self, node: &VarDecl) {
let track = !self.skip_transform_info && !node.declare && node.kind == VarDeclKind::Const;

@chatgpt-codex-connector chatgpt-codex-connector Bot Aug 6, 2026

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P2 Badge Record ambient const initializers

For ambient literal const declarations such as declare const x = 1; enum E { A = x, B }, TypeScript erases the declaration but still classifies x as a constant enum expression and emits A = 1 / B = 2. Because track rejects every declare var decl here, the declaration is stripped while x is never added to const_vars, so the enum is emitted as a runtime read of a non-emitted binding and the following member becomes undefined; allow ambient declare const declarations with constant initializers to populate the semantic map without retaining them.

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for decl in &node.decls {
decl.visit_with(self);

if !track {
continue;
}

let Pat::Ident(BindingIdent { id, type_ann: None }) = &decl.name else {
continue;
};
let Some(init) = &decl.init else { continue };

if !EnumValueComputer::can_fold_shape(init) {
continue;
}

let value = EnumValueComputer {
enum_id: &id.to_id(),
unresolved_ctxt: self.unresolved_ctxt,
record: &self.info.enum_record,

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P2 Badge Respect mutable enum reads in const folding

When tsEnumIsMutable is enabled, enter_expr_for_inline_enum deliberately leaves reads from non-const enums un-inlined, but this new const-variable collection still resolves const x = D.A through enum_record here. In code such as enum D { A = 1 } D.A = 5; const x = D.A; enum E { A = x }, mutable-enum mode keeps x as a runtime read of D.A while the semantic map records x as 1 and emits E.A = 1, changing runtime behavior; skip recording const initializers that depend on non-const enum members when mutable enums are enabled, or thread that option into the evaluator.

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@baltasarblanco Baltasar Blanco (baltasarblanco) Aug 6, 2026

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Good catch — this was a real regression, fixed in 8eeebc5. Confirmed by diffing against main: enum D { A = 1 } (D as any).A = 5; const x = D.A; enum E { A = x } emitted E["A"] = 1 on this branch versus E[E["A"] = x] = "A" on main. The emitted code contradicted itself — const x = D.A was preserved while the enum member used the folded value.

The option is now threaded into the collection pass and mirrors the guard in transform.rs: with mutable enums enabled, only const enum members resolve through a member expression. Both sides are covered in ts_enum_is_mutable_true.

Separately, enum F { A = D.A } (no const in between) still folds under tsEnumIsMutable — that path goes through transform_ts_enum_member, is unchanged by this PR and behaves the same on main. I'll report it separately.

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P2 Badge Include ambient enum members in const folding

When a const initializer reads an ambient enum member, e.g. declare enum A { X = 1 } const x = A.X; enum B { Y = x, Z }, TypeScript treats x as a constant enum expression and emits Y = 1 / Z = 2. This new const-variable collection evaluates against self.info.enum_record, but declare enums are skipped from that record, so x is never inserted into const_vars and B is emitted as a runtime read with the following member becoming undefined; consider recording ambient enum values for semantic folding even though their declarations are removed.

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const_vars: &self.info.const_vars,
const_enum_only: self.ts_enum_is_mutable.then_some(&self.info.const_enum),
}
.compute(init.clone(), EvalCtx::CONST_INIT);

if value.is_const() {
self.info.const_vars.insert(id.to_id(), value);

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P2 Badge Track namespace-qualified const references

When the const is exported from a namespace, e.g. namespace N { export const n = 1 } enum E { A = N.n, B }, TypeScript treats N.n as a constant enum expression and auto-increments B to 2. This only stores the local binding id n, while enum evaluation of N.n remains an opaque member expression, so A is not treated as constant and the following defaulted member falls back to undefined; include the namespace/exported binding in the const lookup or resolve qualified const member expressions.

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@baltasarblanco Baltasar Blanco (baltasarblanco) Aug 6, 2026

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Confirmed against tsc 5.9.3, including the following member collapsing to undefined. Out of scope here: compute_member bails on any non-Ident object, which is an independent change. Opened as #12102.

}
}
}

fn visit_ts_enum_decl(&mut self, node: &TsEnumDecl) {
node.visit_children_with(self);

Expand Down Expand Up @@ -566,6 +607,7 @@ impl Visit for SemanticAnalyzer {
&id.to_id(),
&default_init,
&self.info.enum_record,
&self.info.const_vars,

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P2 Badge Account for later outer consts in nested enums

When an enum is inside a function-like body that appears before an outer const declaration, e.g. function f(){ enum E { A = x, B } } const x = 1, TypeScript still treats x as a constant enum expression and emits A = 1 / B = 2. This pass evaluates the enum immediately with the current const_vars map, so the later outer const has not been recorded yet and the nested enum is emitted as a non-constant member with B becoming undefined; defer or precollect eligible outer-scope const initializers without enabling same-scope forward references.

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@baltasarblanco Baltasar Blanco (baltasarblanco) Aug 6, 2026

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Confirmed against tsc 5.9.3. Matching it needs deferred evaluation of enums nested in function bodies, which is a different rule from the syntactic one this PR implements — top-level forward references must keep their current behavior, since tsc does not fold those either and they are a TDZ error at runtime. Left as-is; it only ever under-folds, which is what main does today. Noted under known limitations.

self.unresolved_ctxt,
self.flow_syntax,
);
Expand Down Expand Up @@ -655,6 +697,7 @@ mod tests {
&id("E"),
&TsEnumRecordValue::Void,
&Default::default(),
&Default::default(),
SyntaxContext::empty(),
true,
);
Expand All @@ -672,6 +715,7 @@ mod tests {
&id("E"),
&TsEnumRecordValue::from(2.0),
&Default::default(),
&Default::default(),
SyntaxContext::empty(),
false,
);
Expand Down
8 changes: 6 additions & 2 deletions crates/swc_ecma_transforms_typescript/src/transform.rs
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Expand Up @@ -22,7 +22,9 @@ use crate::{
retain::{should_retain_module_item, should_retain_stmt},
semantic::SemanticInfo,
shared::enum_member_name,
ts_enum::{static_enum_member_name, EnumValueComputer, TsEnumRecordKey, TsEnumRecordValue},
ts_enum::{
static_enum_member_name, EnumValueComputer, EvalCtx, TsEnumRecordKey, TsEnumRecordValue,
},
utils::{assign_value_to_this_private_prop, assign_value_to_this_prop, Factory},
};

Expand Down Expand Up @@ -1112,6 +1114,8 @@ impl Transform {
enum_id: &id.to_id(),
unresolved_ctxt: self.unresolved_ctxt,
record: &self.semantic.enum_record,
const_vars: &self.semantic.const_vars,
const_enum_only: None,
};

let member_list: Vec<_> = members
Expand All @@ -1133,7 +1137,7 @@ impl Transform {
// references can be rewritten to runtime property
// accesses. Implicit Flow enum members do not have an
// initializer, so keep the semantic value as-is.
let mut recomputed = enum_computer.compute(init);
let mut recomputed = enum_computer.compute(init, EvalCtx::RECOMPUTE);
if let TsEnumRecordValue::Opaque(expr) = &mut recomputed {
expr.visit_mut_with(&mut RefRewriter {
query: EnumMemberRefQuery {
Expand Down
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