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…Script type signatures The type skipper's copy of the array binding grammar skipped commas only before the first element. After an element it consumed one comma, so a second consecutive comma reached the recursive binding skip and failed. In type arguments that failure made bun parse a call such as useState<(([a, , b]: number[]) => number) | null>(null) as a comparison. Treat a comma as an empty element on every iteration, like parse_binding does for array patterns in value positions.
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Updated 2:16 AM PT - Sep 21st, 2026
✅ @robobun, your commit e6d1b559a6c69ef3b42a214781c222ba3acefdec passed in 🧪 To try this PR locally: bunx bun-pr 43673That installs a local version of the PR into your bun-43673 --bun |
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Status: ready for review. How to reproduce the bug on bun 1.4.3: const t = new Bun.Transpiler({ loader: "ts" });
t.transformSync("useState<(([a, , b]: number[]) => number) | null>(null)");
// prints "useState < (([a, , b]) => number) | null > null;\n"
// correct "useState(null);\n"
t.transformSync("let x: ([a, , b]: number[]) => void = y");
// throws "Unexpected ,"
// correct "let x = y;\n"
t.transformSync("type T = { foo([b, , c]: number[]): void }; z");
// throws "Backtrack"
// correct "z;\n"TypeScript 6.0.2 reports no parse diagnostics for these three inputs. Run the new test: bun bd test test/bundler/transpiler/transpiler.test.js -t "holes in array binding patterns"The test fails with |
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Inline comments:
In `@src/js_parser/parse/parse_skip_typescript.rs`:
- Around line 77-84: Update the binding-element loop around
skip_type_script_binding to track whether the current element consumed a rest
marker, and before accepting a following comma, return the same “Unexpected ','
after rest pattern” error used by parse_binding. Preserve normal comma handling
for non-rest bindings and continue skipping valid TypeScript bindings.
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src/js_parser/parse/parse_skip_typescript.rstest/bundler/transpiler/transpiler.test.js
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LGTM — a small, self-contained loosening of the TypeScript type-skipper's array-binding loop that now mirrors the shape parse_binding already uses for value-position patterns.
What was reviewed:
- Loop termination in the rewritten
[arm ofskip_type_script_binding(src/js_parser/parse/parse_skip_typescript.rs:74-91): every iteration either consumes a comma, advances through the recursive binding call (identifier/this/[/{all callnext()), returnsBacktrackfrom the default arm (covers[...], EOF, stray tokens), or breaks on]— no path can spin. - All three callers (
skip_typescript_fn_args, the method-signature arm at line 1112, and the arrow/type-argument backtrackers): the change only widens what the function-type attempt accepts, and the new tests cover the call-vs-comparison and return-type-vs-tuple shapes where that attempt competes with expression parsing. - Negative contract is kept:
[a, , b]as a tuple type still errors with "Unexpected ," (tests at the end of the new block). Test execution was not available in this environment; verification is by reading the code and tracing the base-branch path, which does hit the defaultBacktrackarm on the second comma.
Extended reasoning...
Overview
The PR touches one arm of skip_type_script_binding in src/js_parser/parse/parse_skip_typescript.rs (the T::TOpenBracket case) and adds one it(...) block to test/bundler/transpiler/transpiler.test.js. Previously, the skipper consumed leading commas in a dedicated pre-loop and then required a binding element after every subsequent comma, so a hole after the first element ([a, , b]) reached the recursive call with a , token and returned Backtrack. The rewrite folds the hole handling into the main loop: if the current token is not a comma, optionally skip ... and recurse; then break unless the next token is a comma. This is the same control-flow shape as parse_binding's T::TOpenBracket arm in src/js_parser/parse/mod.rs:1023-1079, minus default-value and rest-comma error handling that the skipper never did.
Security risks
None. This is parser control flow over source text in a type-erasure path; no allocation, no unsafe, no external I/O. The only observable effect is which TypeScript inputs are accepted or how a backtracking attempt resolves. The change strictly widens acceptance inside the skipper; termination is guaranteed because every loop iteration advances the lexer or exits.
Level of scrutiny
Moderate, focused on two questions: can the loop fail to advance, and can wider acceptance in the skipper flip a backtracking heuristic the wrong way. On the first: a comma token is always consumed at the bottom of the loop; a non-comma token goes through skip_type_script_binding, whose identifier/this/[/{ arms all call next() and whose default arm returns an error, so [...], EOF, and stray tokens all exit via ?. On the second: the three callers are skip_typescript_fn_args (function/constructor types and method signatures), and the two backtracking helpers for arrow return types and type arguments. In each, the function-type attempt previously failed on the hole and fell through to a tuple-type or expression interpretation that TypeScript itself rejects or parses differently; the added tests pin the expected outcomes for f<([a, , b]: T) => c>(x) as a call and (p): ([a, , b]) => c => e as a return-type annotation, alongside the already-passing [a, b] variants, which makes the parallel explicit. The remaining ruled-out candidates (accept-to-reject flips through the tuple fallback, the stack-overflow swallowing in lexer_backtracker_bool, ... before every element) are pre-existing behaviors of the skipper or affect only inputs TypeScript also rejects, and are not introduced by this diff.
Other factors
The new test block is placed in the existing transpiler test file per repo convention, uses the file's in-process expectPrinted_/expectParseError helpers, and includes negative cases so the tuple-type path still errors. I could not execute bun bd test or the USE_SYSTEM_BUN=1 failure check in this environment, so the "fails on the base" claim is confirmed by tracing the old code rather than by running it; the trace is unambiguous (the second comma reaches the default arm and returns Backtrack). Neither file is covered by .github/CODEOWNERS. No prior review activity or outstanding objections exist on the timeline.
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Problem
useState<(([a, , b]: number[]) => number) | null>(null)printsuseState < (([a, , b]) => number) | null > null, so the call never runs. TypeScript printsuseState(null).let x: ([a, , b]: number[]) => voidgivesUnexpected ,, and{ foo([b, , c]: number[]): void }givesBacktrack. TypeScript accepts both.[arm ofskip_type_script_binding(src/js_parser/parse/parse_skip_typescript.rs:71). It skips commas only before the first element. After an element, a second comma reaches the recursive call and fails.Fix
parse_binding(src/js_parser/parse/mod.rs:1023) already has this loop shape.].lexer.unexpected()does nothing while error logging is off, soy = (p): [a, , b] => cstill passes. js_parser: validate TypeScript class member modifiers like esbuild #40817 changes that mechanism.test/bundler/transpiler/transpiler.test.jsfails on bun 1.4.3. Self-reviewed: 6 requests, 3 addressed. Not followed: 3 requests to fold this into js_parser: skip computed keys in destructured parameters of TypeScript function types #38890, js_parser: reportUnexpected <token>instead ofBacktrackfor a bad parameter in a type literal signature #43634 or an initializer change (reasons in Notes).Background
parse_skip_typescript.rs) skips type annotations, because bun erases types.skip_type_script_bindingis its copy of the destructuring grammar, for a parameter such as([a, b]: T) => void.<as a comparison, or( ... )as a parenthesized type.Notes
How the wrong output happens. There are two shapes, and both start with the function type attempt that fails at the hole.
f<T, ([a, , b]: T) => c>(x)becomes the sequence expressionf < T, ([a, , b]) => c > x. As a statement, bun drops the unused arrow function and printsf < T;.[a, , b]as a tuple type.lexer.unexpected()only logs, and error logging is off, so the tuple skip passes over the hole. Bun then takes([a, , b])as the return type andc => eas the body.y = (p): ([a, , b]) => c => eprintsy = (p) => (c) => e. TypeScript printsy = (p) => e.With the fix the function type attempt succeeds in both shapes, which is what bun already does for
[a, b].Comparison with TypeScript 6.0.2. I ran 125 inputs through
ts.createSourceFile(...).parseDiagnosticsplusts.transpileModule, and throughBun.Transpiler. Bun 1.4.3 agrees with TypeScript on 44 rows. The fixed build agrees on 114 rows. On 1.4.3, 12 rows print the wrong code: 3 in type arguments and 9 in arrow function return types (for exampleclass C { m = (p): ([a, , b]) => c => e }andy = (p): p is ([a, , b]) => c => e). The fixed build prints the same code as TypeScript for all 12.Inputs that change from accepted to rejected.
y = (p): ([a, , b]) => cprintedy = (p) => c, andy = (p): ([a, , b]) => { return 1 }printedy = (p) => { return 1; }, through the tuple fallback. Both are nowExpected ";" but found ":". TypeScript reports TS1005 at the same:. Bun already gives this error for the same inputs written with[a, b]. No valid TypeScript program takes this path, because a tuple type cannot have a hole.Non-goals: the 11 rows that still differ. None of them is about a hole in the binding skipper.
([a = 1, , b]: T) => void). Bun rejects an initializer there for every pattern form today. TypeScript parses it and reports TS2371 later.thisinside a nested pattern (([this, , b]: T) => void). TypeScript rejects them. Bun acceptsthisat every depth today (([this, b]: T) => voidpasses on 1.4.3), so these two rows change from rejected to accepted.(p: [a, , b]) => voidandy = (p): [a, , b] => cpass on 1.4.3, and([a, , b]: [number, , string]) => voidchanges from rejected to accepted. This is thelexer.unexpected()mechanism. Bun 1.4.3 also acceptslet x: (p: ,) => void = yandf<,>(x)through it. TypeScript reports TS1110 for each. js_parser: validate TypeScript class member modifiers like esbuild #40817 adds the missingreturn Err(SyntaxError)after those calls.a ? (b) : ([c, , d]) => eprintsa && b;with and without the hole, andnew Map<...>()printsnew Map;.Self-review. The review asked for three changes, and all three are in this PR: the test block moved off the position that #43634 uses, the related PRs and non-goals are listed here, and the body leads with the type argument case, says how the bug was found, and scopes the tuple claim. Three more requests asked to fold this change into another one. I did not follow them:
Unexpected <token>instead ofBacktrackfor a bad parameter in a type literal signature #43634 or js_parser: validate TypeScript class member modifiers like esbuild #40817: those PRs make the skipper reject more. This PR only makes it accept more, so it can land alone.Related open PRs on the same function. Each one edits a different arm, and the Rust hunks do not overlap.
{arm.{arm.Unexpected <token>instead ofBacktrackfor a bad parameter in a type literal signature #43634: the default arm reportsUnexpected <token>in place ofBacktrack. After it lands, an invalid parameter in a method signature gets a real message. The hole inputs of this PR no longer reach that arm.lexer.unexpected().The test block sits below "non-null assertion with new operator", because #43634 adds its block directly below "types". esbuild's
skipTypeScriptBinding(internal/js_parser/ts_parser.goat evanw/esbuild@f6058f8364) has the same loop as the old code, so there is nothing to port.Suites run with the debug build (
bun bd test).test/bundler/transpiler/transpiler.test.js: 223 pass, 0 fail.transpiler-stack-overflow,decorator-metadata,decorators,es-decorators,runtime-transpiler,property,scope-mismatch-panicintest/bundler/transpiler/, plustest/bundler/esbuild/ts.test.ts: 540 pass, 0 fail.[human-review] gate passed · iteration 0 · 2 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