[robobun]

Stacked on #45915 and #45557. No user reported this.

Problem

• With no legacy decorator option in tsconfig.json, class E { accessor a = 1 } in bun repl gives SyntaxError: Unexpected identifier 'a'. Expected a ';' following a class field. Decorators get the legacy arguments. A file runs both.
• Repl::transform_for_repl (src/runtime/cli/repl.rs:2149) passed constants, not the root tsconfig.json values that the module loader passes.

Fix

• The REPL passes experimentalDecorators and emitDecoratorMetadata of the root tsconfig.json, through the TsconfigOptions argument of parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557.
• In replMode, a class declaration with decorators reports the class (src/js_parser/repl_transforms.rs:900).
• Verified: test/js/bun/repl/repl.test.ts (11 new) and test/js/bun/transpiler/repl-transform.test.ts (2 new), all failing on the base. Also the decorator suites.
• Self-reviewed: 26 concerns raised, 23 addressed (Notes).

Background

• TC39 decorators are the default (feat(transpiler): implement TC39 standard ES decorators lowering #26436) and include accessor. A TypeScript file gets the legacy ones when tsconfig.json sets experimentalDecorators or emitDecoratorMetadata.
• A TC39 class declares temporaries. On main the REPL makes them globals, so a second class breaks the first (replMode: keep helper imports and lowering temporaries in the wrapper #45915).
• Considered three assignments after ParserOptions::init, without parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557: a fourth copy of the formula. It is the fallback.

Downsides

• Changed at the prompt without those options: a decorator gets (value, context), and a parameter decorator is not called (js_parser: reject parameter decorators when experimental decorators are off #43321). experimentalDecorators in tsconfig.json restores the legacy ones.
• Cost (release builds): +3 instructions per input, +1 per class declaration in any file, 0 allocations, 0 syscalls, .text +512 bytes.
• Unlike a file: .load x.js is parsed as TSX (Notes).

Notes

The stack

• The base branch robobun/64f2d4c5/stack-base is main a8b5e920c4 plus a merge of replMode: keep helper imports and lowering temporaries in the wrapper #45915 and a merge of parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557. This PR is the two commits on top. I retarget it to main when both are merged.
• replMode: keep helper imports and lowering temporaries in the wrapper #45915 is the hard prerequisite. This PR needs its rule for declarations (repl_binding_persists in src/js_parser/repl_transforms.rs). With that rule switched off on a scratch build, the two sessions of the new tests a class with ... is not changed by a later one give [ 201, 202 ] and TypeError: Cannot read from private field.
• parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557 is the carrier. If it is declined, the REPL sets the three options after ParserOptions::init.
• Hardening follow-up: fix regressions from #46021 and more crashes #46270 (merged) was the third prerequisite. With emitDecoratorMetadata, the input true ? (): new () => object => { class A { @dec p: Date } return A } : null stopped the parser with panic: index out of bounds. A file had that panic on 1.4.3. The new tests evaluate this input at the prompt and in a file.

What changes at the prompt

An input with no decorator and no accessor field does not change: the text that the REPL evaluates is byte-identical for 50 such inputs in 3 tsconfig states.

Root tsconfig.json of the working directory	1.4.3	This PR	A .ts file, 1.4.3 and this PR
none, or neither option	legacy, accessor is a SyntaxError	TC39	TC39
experimentalDecorators	legacy	legacy	legacy
emitDecoratorMetadata	legacy, no metadata	legacy, with metadata	legacy, with metadata
sets the option and extends another file (#30477)	legacy	TC39	TC39
is in a directory above (#28605)	legacy	TC39	TC39

• The last two rows are defects of the tsconfig loader (Bun 1.3.10 breaks --tsconfig-override flag #30477, jsxImportSource from nested tsconfig ignored when running from workspace root #28605). Open PRs touch them (resolver: support the tsconfig extends array form #43110, Transpile each file with the tsconfig.json nearest to it at runtime #38719). The prompt gets what a file gets there, and the new tests assert only that. A flat tsconfig.json, or bun repl --tsconfig-override with a flat file, keeps the legacy decorators.
• Where the tsconfig.json sets experimentalDecorators, accessor is still a SyntaxError at the prompt, as in a file (Decorators on accessor class fields fail to parse in Bun #29197, Accept and lower the accessor keyword in TypeScript files using experimentalDecorators #38125).
• A decorator written for the legacy arguments fails at the prompt where no option is set: one that reads descriptor.value throws TypeError: undefined is not an object. That is what the same decorator does in a file there.
• A class declaration with decorators now reports the class in both implementations. Before: @dec class D {} reported undefined with TC39 decorators. With the legacy ones, class Q { @dec m() {} } reported a property descriptor and class T { @dec p = 1 } reported undefined. This also changes the output of Bun.Transpiler({ replMode: true }) for those inputs: one statement that reads the class name is added.

Why the default changes without a flag

• feat(transpiler): implement TC39 standard ES decorators lowering #26436 made TC39 decorators the default and the tsconfig option the switch. fix(transpiler): pass experimentalDecorators/emitDecoratorMetadata to Bun.Transpiler parse options #27582 did the same for Bun.Transpiler after a user report (experimentalDecorators: true in tsconfig has no effect — Bun.Transpiler and bun build always emit TC39 decorators #27575).
• The native REPL (feat(repl): implement native Zig REPL with full TUI support #26304) never set the option. Its code has no mention of decorators, so the legacy decorators at the prompt were not a decision.
• The option in tsconfig.json is the switch that a file uses. A new flag would be a second one.

Not in this PR

• .load x.js, and every input, is parsed as TSX. Where the tsconfig.json sets a legacy option, a .js file gets TC39 decorators and the prompt gets the legacy ones. That is the same as before.
• A class with a decorator or an accessor field that is evaluated more than once (a loop body, a parameter default) shares its temporaries, as in a file (js_parser: declare decorator lowering temporaries per iteration inside loops (stacked on #38734) #38933, js_parser: declare decorator lowering temporaries per evaluation in parameter defaults and field initializers #38904).
• A decorator on its own line before the class is evaluated alone, because the prompt continues only on open brackets.
• bun repl still ignores --feature and --no-macros.
• Derive decorator options from tsconfig in scanImports and the REPL #33384 made this switch in July and was closed by the stale sweep. It did not know about the shared temporaries.

Measurements

Release builds (ThinLTO) of the base and of this diff on main 0793250a5b9, in one build directory. The source diff has not changed since. Tools: llvm-size, llvm-nm -S, qemu-x86_64 -one-insn-per-tb -d exec,nochain -dfilter (instructions inside one function), qemu-x86_64 -strace, gdb 16.3 (breakpoints on 16 mi_* entry points, and a dump at Bun__REPL__evaluate). perf, valgrind, strace and bloaty are not installed.

• .text 81,989,093 to 81,989,605 bytes (+512). .rodata, .data and .bss do not change. The stripped binary is 107,583,200 bytes in both.
• 5 of 106,985 function symbols differ: Repl::transform_for_repl 3,214 to 3,230 bytes, s_class 1,205 to 1,239 (two parser instantiations), and the new cold repl_report_class_declaration, 255 bytes (two instantiations). Options::init, scanImports, parse_worker::run_from_thread_pool and Transpiler::parse are byte-identical.
• bun repl, per input: 322 to 325 instructions in transform_for_repl, for 2 inputs in 3 tsconfig states. Options::init: 75 to 75.
• bun run, per class declaration, in s_class: 75 to 76 instructions (a class with no decorator, with a member decorator, with an accessor field, in .ts and .js). 84 to 83 for a class with a class decorator.
• Allocator calls inside transform_for_repl: 2,366 to 2,366 for a session of 50 inputs without decorators. Syscalls of that session: 705 to 705, and 709 to 709 with a tsconfig.json (1 open of it in both).
• An input with a method decorator, where no option is set: 95 to 139 allocator calls and +2.6 syscalls (clock_gettime). An input with an accessor field, which was a SyntaxError: 38 to 124 allocator calls. At a terminal it costs +18 syscalls (7 ioctl, 2 rt_sigaction), because it is awaited like every input that imports a helper.
• Runtime transpiler cache: 0 of 12 entries differ (diff -r over js, jsx, ts and tsx modules with decorators, under no option, experimentalDecorators, and both).
• Bun.Transpiler({ replMode: true }) output for 67 inputs: 6 differ with TC39 decorators and 10 with the legacy ones, by 0 to 8 bytes. All are top-level class declarations with decorators.

Tests

• repl.test.ts: accessor and TC39 decorators with no tsconfig.json, two lowered classes in one session (two tests), a decorator imported from a module, .load, .copy, -p, and five directory shapes where the prompt and bun program.ts run the same program and must agree.
• repl-transform.test.ts: a class declaration reports the class with TC39 and with legacy decorators, and only in replMode at the top level. With repl_mode deleted from the guard on a scratch build, both tests fail.
• On a debug build of the base source: the new REPL tests and the 2 new replMode tests (4 with both transform methods) fail, and the other tests of the two files pass.
• On a debug build of this PR: repl.test.ts 148 pass, repl-transform.test.ts 70 pass, transpiler.test.js 307, es-decorators.test.ts 427, es-decorators-esbuild.test.ts 146, decorators.test.ts 25, decorator-metadata.test.ts 5, runtime-transpiler.test.ts 15, test/regression/issue/44915.test.ts, test/cli/test/coverage.test.ts 43. test/internal/source-lints passes with the release binary.
• Windows was not run locally. The change has no platform branch.

Self-review

26 concerns raised, 23 addressed. The two largest: the echo of a class declaration covered only TC39 decorators and moved to the statement visitor, and the metadata option waited for #46270. Not taken:

• Base the switch on replMode: keep helper imports and lowering temporaries in the wrapper #45915 alone, with assignments in place of the argument of parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557. The design review chose the argument because the formula then stays in one function. It is the stated fallback.
• File an issue for the defect. This PR is the record.
• A test that pins .load x.js under a legacy tsconfig.json. It would pin a mismatch.

[robobun]

^{Updated 1:04 AM PT - Oct 24th, 2026}

✅ @robobun, your commit 0d7671138aa735f915238432e4556fca22954e7d passed in Build #131697! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 46457

That installs a local version of the PR into your bun-46457 executable, so you can run:

bun-46457 --bun

[robobun]

Status

• Reproduced on 1.4.3 and on main. In an empty directory, printf 'class E { accessor a = 1 }\nnew E().a\n.exit\n' | bun repl prints SyntaxError: Unexpected identifier 'a'. Expected a ';' following a class field. The same two lines in a .ts file print 1.
• This PR is a draft. Its base branch is main plus replMode: keep helper imports and lowering temporaries in the wrapper #45915 and parser: require the tsconfig options in ParserOptions::init, fix scanImports on decorators and accessor fields #45557, and it cannot merge before them. I retarget it to main when both are merged.