Implement the replacer argument of YAML, TOML and JSON5 stringify

[robobun]

Problem

• Bun.YAML.stringify, Bun.TOML.stringify and Bun.JSON5.stringify throw X.stringify does not support the replacer argument for any non-null replacer (src/runtime/api/YAMLObject.rs:34 and siblings).
• YAML read every property twice (anchors, then the write) and TOML read each table twice, so a function replacer called there would run twice.

Fix

• New src/runtime/api/stringify_replacer.rs. Replacer reads the argument and calls a function replacer as JSON.stringify does. Properties yields the own enumerable properties, or the listed ones in list order with one [[Get]] each.
• JSON5 calls the replacer inline in its one pass. TOML and YAML read the value once, depth first, in JSON.stringify order, into native records (Value in TOML, Node in YAML). The write pass runs no JS. Getters now run once in YAML (two upstream tests updated).
• A function replacer runs once per occurrence of a shared object, as in JSON.stringify, so YAML writes such an object once per key. A cycle still gets an anchor.
• Verified: test/js/bun/json5/json5.test.ts (compared call by call with JSON.stringify), yaml.test.ts, toml.test.ts. All new tests fail without this change. Also bun-types, source lints, clippy.

Background

• A replacer is the second argument of JSON.stringify. A function gets every (key, value) pair, with the holder as this, and returns what to write. An array lists the names to write.
• A JSValue on the Rust heap is not a GC root. The records are live while the replacer runs JS, so every value they hold also goes into a MarkedArgumentBuffer, which the GC marks.
• JSC__JSValue__getMayBeIndex (next to putMayBeIndex) is the [[Get]] by any name that a key list needs.

Notes

• Behavior matches JSON.stringify call for call on the inputs tried (call order, keys, holders, values, result), with two exceptions: no toJSON is consulted (unchanged, and deliberately so: a format-specific hook would be its own design), and Symbol-keyed properties are written by description, which the stringifiers already did and JSPropertyIterator: enumerate string keys only, never symbols #37010 fixes.
• JSValue::get is the options-bag accessor: it skips index-like names and stops before Object.prototype. get_may_be_index is the [[Get]] counterpart of put_may_be_index: any name, whole prototype chain, undefined when missing. The unused JSC__JSValue__getPropertyValue is left alone, because Remove dead code from the JSC FFI glue, WebCore bindings, usockets, built-in JS, and orphaned scripts #39618 removes it.
• Without a replacer the stringifiers do the same JS work as before, or less: Replacer::from_js does two type checks and Properties::init wraps the same JSPropertyIterator. YAML reads each property once instead of twice. In the debug build, YAML of a 300k element array went from 2388 ms to 1083 ms. With a function replacer, the three stringifiers cost about 1.9x the function-replacer path of JSON.stringify per element.
• YAML with a function replacer: known_collections holds only the collections on the current path, so a repeat elsewhere is read again (and the replacer called again), as JSON.stringify does. Without a function replacer it holds every collection, so a repeat becomes an alias as before.
• TOML reads null, undefined, a symbol or a function in an array as an error at read time, so a replacer that returns one of them fails with the same message as before.
• YAML.stringify is declared string | undefined like its siblings, since a replacer can drop the root. This is types: fix YAML.stringify return type #34162, credited in the commit.
• Rebased onto Make bun_core::String own its WTF ref #40238 (bun_core::String owns its WTF ref). The three stringifiers now hold String values directly (OwnedString is gone), Properties::next yields a StringView like JSPropertyIterator::next, and TOML takes main's parent-linked Path for table headers instead of a Vec. A 256 KiB-string RSS probe over all three stringifiers, with and without replacers, shows no growth.
• A replacer that is neither callable nor an array is ignored, as with JSON.stringify. The array check is JSValue::is_array_including_proxy, a wrapper of JSC::isArray (the spec IsArray), so a Proxy of an array is a key list too. The .d.ts accepts a function, an array or null.
• Every example in the docs and the JSDoc was run against the debug build. The probes also ran under BUN_JSC_validateExceptionChecks=1.
• types: accept binary input in YAML.parse and JSON5.parse #34080 also adds test/integration/bun-types/fixture/json5.ts. Whichever lands second keeps both sets of lines.

---

no test proof · iteration 6 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/json5/json5.test.ts

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Reviewing files that changed from the base of the PR and between 0823e50 and d943cd6.

📒 Files selected for processing (17)

• docs/runtime/json5.mdx
• docs/runtime/toml.mdx
• docs/runtime/yaml.mdx
• packages/bun-types/bun.d.ts
• src/jsc/JSValue.rs
• src/jsc/bindings/bindings.cpp
• src/runtime/api.rs
• src/runtime/api/JSON5Object.rs
• src/runtime/api/TOMLObject.rs
• src/runtime/api/YAMLObject.rs
• src/runtime/api/stringify_replacer.rs
• test/integration/bun-types/fixture/json5.ts
• test/integration/bun-types/fixture/toml.ts
• test/integration/bun-types/fixture/yaml.ts
• test/js/bun/json5/json5.test.ts
• test/js/bun/toml/toml.test.ts
• test/js/bun/yaml/yaml.test.ts

Walkthrough

Changes

JSON5, TOML, and YAML stringifiers now support function and property-list replacers through a shared implementation. Public types, runtime documentation, and tests cover filtering, transformation, omission, ordering, cycles, aliases, and format-specific values.

Suggested reviewers: alii, dylan-conway, jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description explains the problem, implementation, behavior, and verification, although it does not use the template headings exactly.
Title check	✅ Passed	The title clearly and concisely identifies the main change: replacer support for YAML, TOML, and JSON5 stringify APIs.

---

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[robobun]

Status: ready for review.

Origin: @dylan-conway asked for the replacer argument of the YAML, TOML and JSON5 stringify functions, then for the behavior to match JSON.stringify apart from toJSON, then for a single pass over the value with no extra JS work.

The current head reads the value once. JSON5 calls the replacer inline. TOML and YAML read the value once, in JSON.stringify order, into native records and write from those records with no JS lookups. Getters run once in YAML now, so two upstream YAML tests that counted two reads were updated.

Verified on the debug build: the new tests in test/js/bun/json5/json5.test.ts, test/js/bun/yaml/yaml.test.ts and test/js/bun/toml/toml.test.ts fail without this change and pass with it. The JSON5 suite compares the replacer protocol call by call with JSON.stringify. The three test directories, test/integration/bun-types, test/internal/source-lints and cargo clippy on bun_jsc and bun_runtime pass.

Includes the YAML.stringify return type change of #34162, credited in the first commit. The branch is rebased onto main (0823e50, after #40238 made bun_core::String own its ref); the stringifiers were ported to that API, and the single-read design replaces the upstream YAML change #39980.

[claude]

I reviewed this PR and didn't find any bugs. Because it adds a new user-facing API feature backed by ~200 lines of new GC-sensitive native code (JSValues held in a Rust HashMap across replacer callbacks, rooted via MarkedArgumentBuffer) plus a new C++ binding, a human look at the design and memory-safety choices would still be worthwhile.

What was reviewed:

• stringify_replacer.rs: the copy-then-serialize walk, cycle/shared-object handling via the copies map, GC rooting of map entries in MarkedArgumentBuffer, stack-overflow guard, and is_leaf for TOML dates.
• The new JSC__JSValue__getMayBeIndex C++ binding — throw scope declared, RETURN_IF_EXCEPTION after get; confirmed the removed JSC__JSValue__getPropertyValue had no other callers.
• Replacer-array parsing against ECMA-262 step 4.b (string/number/wrapper accepted, dedup); documented divergences (index-key ordering, single visit of shared objects) are stated in the PR notes and covered by tests.

Extended reasoning...

Overview

This PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify, and Bun.JSON5.stringify, which previously threw. It adds a shared src/runtime/api/stringify_replacer.rs (~200 lines) that pre-walks the input, invokes the replacer once per property with the JSON.stringify protocol, and builds a plain-object copy that the existing stringifiers then serialize unchanged. Supporting changes: a new JSC__JSValue__getMayBeIndex C++ binding (replacing the unused JSC__JSValue__getPropertyValue) plus its Rust wrapper, an is_date_time helper factored out of TOMLObject.rs, updated .d.ts signatures and JSDoc, three docs pages, three type-fixture files, and ~400 lines of new tests across the three format test files.

Security risks

No auth/crypto/permissions surface. The replacer is user JS invoked from native code, which is the usual re-entrancy hazard: a callback can allocate, GC, or throw. The implementation addresses this by rooting every original+copy pair in a MarkedArgumentBuffer before recursing, propagating JsResult from every function.call/get_may_be_index/put_*, and guarding recursion with StackCheck. The property-list path performs a full-prototype-chain [[Get]], matching JSON.stringify, so no new prototype-pollution vector beyond what JSON.stringify already has. I did not identify a concrete exploit path.

Level of scrutiny

High. This is new user-facing API surface (per REVIEW.md, API-design changes warrant maintainer agreement) implemented in native code that holds JSValues on the Rust heap across calls that re-enter JS — precisely the class REVIEW.md flags as most-blocked. The copy-then-serialize design also encodes two documented divergences from JSON.stringify (index-like keys sort first under a property list; a shared object's children are replaced once). Those are reasonable and tested, but they are API contract decisions a maintainer should sign off on.

Other factors

Test coverage is thorough: the JSON5 suite checks the replacer protocol against JSON.stringify on the same input, exercises cycles, shared objects, boxed primitives, thrown exceptions, callable Proxies, deep-nesting RangeError, and a GC-stress test that calls Bun.gc(true) mid-walk; YAML and TOML add format-specific coverage (anchors/aliases preserved, TOML layout follows replaced values, Date/Temporal remain leaves). The removed C++ function has no other references in the tree. The no-replacer path is unchanged apart from two type checks in from_js. Given the scope — new API, new native module with GC lifetime reasoning, new FFI entry point — this exceeds the "simple/mechanical" bar for auto-approval.

[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@packages/bun-types/bun.d.ts`:
- Around line 1312-1316: Update YAML.stringify’s return type and its `@returns`
documentation to string | undefined, matching TOML.stringify and JSON5.stringify
and the runtime behavior. Also update the YAML type fixture assertion from
string to string | undefined.

In `@test/js/bun/toml/toml.test.ts`:
- Around line 892-894: Update the test named “space is accepted and ignored” to
assert the expected literal TOML document text directly, rather than comparing
two TOML.stringify calls. Preserve verification that the space argument is
accepted while ensuring the assertion can fail if stringify produces an
incorrect or undefined result.

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📒 Files selected for processing (17)

• docs/runtime/json5.mdx
• docs/runtime/toml.mdx
• docs/runtime/yaml.mdx
• packages/bun-types/bun.d.ts
• src/jsc/JSValue.rs
• src/jsc/bindings/bindings.cpp
• src/runtime/api.rs
• src/runtime/api/JSON5Object.rs
• src/runtime/api/TOMLObject.rs
• src/runtime/api/YAMLObject.rs
• src/runtime/api/stringify_replacer.rs
• test/integration/bun-types/fixture/json5.ts
• test/integration/bun-types/fixture/toml.ts
• test/integration/bun-types/fixture/yaml.ts
• test/js/bun/json5/json5.test.ts
• test/js/bun/toml/toml.test.ts
• test/js/bun/yaml/yaml.test.ts

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[claude]

I reviewed this PR and didn't find any bugs. It adds a new user-facing API feature backed by ~200 lines of GC-sensitive native code (JSValues held in a Rust-heap HashMap rooted via MarkedArgumentBuffer while user JS runs) plus a new C++ FFI binding, and documents two intentional divergences from JSON.stringify semantics — a human should sign off on the API shape and the GC-rooting approach. The comment-cop bot also left ~15 inline flags on doc comments that haven't been addressed yet.

What was reviewed:

• stringify_replacer.rs: exception propagation on every ? boundary, stack-overflow guard, MarkedArgumentBuffer rooting of both original and copy before recursing (cycles/shared objects stay shared).
• JSC__JSValue__getMayBeIndex: throw-scope + RETURN_IF_EXCEPTION present; zero_is_throw matches the encoded-zero error return.
• TOML is_date_time refactor: same predicate now backs the copier's leaf check and all three existing call sites, so they can't drift.
• Tests cover the replacer protocol against JSON.stringify, GC stress with Bun.gc(true) mid-walk, deep-nesting RangeError, and per-format rules (YAML anchors, TOML null/undefined errors).

Extended reasoning...

Overview

This PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify, and Bun.JSON5.stringify, which previously threw "does not support the replacer argument". The core is a new src/runtime/api/stringify_replacer.rs (~200 lines) that walks the input once, calls the replacer per property with the JSON.stringify protocol, and builds a plain JS-object copy that each stringifier then serializes unchanged. Supporting changes: a new C++ binding JSC__JSValue__getMayBeIndex (the [[Get]] counterpart of the existing putMayBeIndex), a shared is_date_time helper in TOMLObject.rs, updated .d.ts signatures and JSDoc, three docs pages, three type-fixture files, and ~350 lines of new tests across the three format test files.

Security risks

None identified. The replacer runs user-supplied JS, but that's the point of the API and matches JSON.stringify. Input is JS values already in the caller's control; there's no parsing of untrusted bytes, no filesystem/network I/O, and no privilege boundary crossed. The main risk class here is memory safety (GC-rooting of JSValues held on the Rust heap while user JS runs), which the PR addresses with MarkedArgumentBuffer and a GC-stress test — but this is exactly the pattern REVIEW.md flags as the most-blocked category and deserves human eyes.

Level of scrutiny

High. This is new user-facing API surface on three public functions, backed by native code that holds JSValues in a Rust HashMap across calls that re-enter JS (the replacer function, to_bun_string, property iteration). The PR description documents two intentional divergences from JSON.stringify (index-like keys sort first in property-list mode; shared objects have their children replaced once, not once per occurrence) — these are API-design decisions a maintainer should ratify. The copy-then-stringify architecture itself is a design choice (vs. threading the replacer through each stringifier) that trades an extra allocation pass for keeping the stringifiers unchanged; reasonable, but worth a human confirming.

Other factors

• The comment-cop github-actions[bot] left ~15 inline "paragraph-long comment" flags on doc comments in the new code that have not been addressed. They look like false positives (the flagged comments are doc comments explaining behavior and citing ECMA-262, not workaround justifications), but they're outstanding automated feedback on the PR.
• Test coverage is thorough: the JSON5 suite checks call order/holders/values against JSON.stringify on the same input, and each format's suite covers its format-specific rules (YAML anchors/aliases survive, TOML layout follows replaced values, TOML null/undefined-in-array still throw). There's a GC-stress test (Bun.gc(true) inside the replacer) and a 200k-deep RangeError test.
• The bug-hunting system found nothing; I also traced exception propagation through from_js/apply/copy_object/copy_array and the new C++ binding and didn't spot missing ?/RETURN_IF_EXCEPTION sites.
• Per the robobun comment, dylan-conway requested this feature, so there's maintainer buy-in on the feature existing — but the specific implementation approach and the documented spec divergences should still get explicit sign-off.

[robobun]

^{Updated 8:48 PM PT - Aug 24th, 2026}

✅ @robobun, your commit d943cd696a3aec827afa3e663484b5d02bc8564e passed in Build #105282! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 39925

That installs a local version of the PR into your bun-39925 executable, so you can run:

bun-39925 --bun

[claude]

I reviewed this PR again after eed1411 and didn't find any bugs — the Proxy-of-array fix (is_array_including_proxy + revoked-Proxy TypeError) and the earlier return-type / literal-assertion changes look correct. Given the scope — new user-facing API across three stringifiers, the read-into-native-records refactor of YAML/TOML, and GC-sensitive MarkedArgumentBuffer rooting of heap-held JSValues — a human sign-off is still worthwhile.

What was reviewed

• Replacer::from_js / Properties: key-list dedup, ToString exception propagation, get_may_be_index for index-like names, spec-IsArray for Proxy.
• GC safety: every Value::Scalar / string / collection stored in the TOML/YAML native records is also appended to the MarkedArgumentBuffer before more JS runs; JSON5's single-pass path keeps values on the native stack.
• YAML behavior change: shared objects with a function replacer are now written per-key (matching JSON.stringify) while cycles keep anchors; block-style write_object empty-case moved to the pre-filtered items.is_empty() check.
• TOML: path-based visiting still catches cycles after the read/write split; write_inline fallthrough is unreachable by construction.

Extended reasoning...

Overview

This PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify, and Bun.JSON5.stringify, matching JSON.stringify's replacer protocol (function or property-list array). It adds a shared stringify_replacer.rs module (Replacer + Properties), two new C++ bindings (getMayBeIndex, isArrayIncludingProxy), and refactors TOML and YAML from walk-and-write to read-once-into-native-records-then-write so the replacer/getters run exactly once. ~1200 lines across 17 files, with docs, .d.ts, type fixtures, and extensive tests in all three suites (including a call-by-call comparison against JSON.stringify).

Prior review

I left two inline findings on earlier revisions: the dead JSC__JSValue__getPropertyValue (author deferred to #39618, which owns those lines — reasonable) and is_array() not unwrapping Proxy targets (fixed in eed1411 with is_array_including_proxy, plus a test covering the trap-reading and revoked-Proxy TypeError paths). Both are resolved. The bug-hunting system found nothing on the current head.

Security risks

None identified. The replacer runs user JS during serialization, but that was already true of getters; the new code correctly propagates thrown exceptions via ? on every .call() / to_bun_string() / get_may_be_index(), and the C++ getMayBeIndex has RETURN_IF_EXCEPTION under a throw scope. The property-list path uses [[Get]] (walks the prototype chain) exactly as JSON.stringify does.

Level of scrutiny

High. This is a feature PR that (a) adds user-facing API surface documented as behaving like JSON.stringify, (b) restructures two existing stringifiers, (c) stores JSValues in Rust heap containers across JS re-entry, and (d) subtly changes existing YAML output when a function replacer is present (shared objects lose aliasing). The GC-rooting design looks sound — every heap-held JSValue (Item::Scalar strings, collection keys in known_collections, TOML Value::Scalar) is pushed to a MarkedArgumentBuffer whose lifetime spans the whole stringify — but this is exactly the kind of code REVIEW.md flags for careful human review.

Other factors

Test coverage is thorough (protocol parity with JSON.stringify, GC stress with Bun.gc(true) mid-walk, stack-overflow bounds, Proxy/revoked-Proxy, exception propagation, index-like keys, non-ASCII names). The one CI failure so far is "step failed outside runner" on macOS aarch64, which reads as infrastructure. The feature was requested by a maintainer (per robobun's status comment), so the design direction has buy-in; human review should focus on the YAML alias-vs-repeat semantics with a function replacer and the MarkedArgumentBuffer rooting completeness.

[claude]

I reviewed this PR and the automated bug-hunting pass found no issues; my two earlier inline nits have been addressed. Because this is a large change — a new shared Replacer/Properties module, restructuring the YAML and TOML stringifiers from two passes to a read-once-into-native-records design, new JSC bindings, and MarkedArgumentBuffer-based GC rooting of heap-stored JSValues — a human look would still be worthwhile.

What was reviewed:

• GC safety of Value::Scalar / Item::Scalar JSValues on the Rust heap — every scalar and collection is appended to the MarkedArgumentBuffer before further JS runs.
• Exception propagation through getMayBeIndex, isArrayIncludingProxy, and the replacer .call() sites — all use ? / RETURN_IF_EXCEPTION.
• YAML anchor/alias behavior with a function replacer (shared objects written once per key, cycles keep anchors) — matches the docs and tests; known_collections.remove after reading is what enables it.
• The write_inline fallthrough in TOML — read filters null/undefined/symbol/function in tables and errors on them in arrays, so the trailing err_in_array is unreachable for a table scalar.

Extended reasoning...

Overview

The PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify, and Bun.JSON5.stringify, matching the JSON.stringify replacer protocol (function or property-list array). It introduces src/runtime/api/stringify_replacer.rs with shared Replacer and Properties types, adds two JSC bindings (JSC__JSValue__getMayBeIndex for [[Get]] by any name and JSC__JSValue__isArrayIncludingProxy for spec IsArray), and restructures the YAML and TOML stringifiers from their previous two-pass designs (YAML: anchor-scan then write; TOML: classify then emit) into a single read pass that runs the replacer and getters once per property, materializing native Node/Value records, followed by a write pass that runs no JS. JSON5 keeps its single pass and calls the replacer inline. Docs, .d.ts types (including the YAML.stringify return-type change to string | undefined), and ~350 lines of new tests across the three suites are included.

Security risks

None identified. This is serialization of in-process JS values to strings; no filesystem, network, credential, or auth surface. The only untrusted-input path is the replacer array's elements being coerced via to_bun_string, which is exception-checked. Prototype pollution is not a concern here (no writes to user objects; the { "": value } root holder is a fresh object).

Level of scrutiny

High. The YAML/TOML restructure is not a mechanical change: it moves cycle detection, anchor assignment, and layout classification into a new read phase, stores JSValues on the Rust heap (in Vec<Node>, Vec<Value>, HashMap<JSValue, NodeId>), and relies on a MarkedArgumentBuffer closure to keep them rooted while the replacer or a getter runs JS. The design intentionally changes YAML's shared-object output under a function replacer (from anchor+alias to per-key expansion, matching JSON.stringify). These are exactly the kinds of memory-safety and behavior-shaping decisions the review guidelines flag for human sign-off. The maintainer who requested the feature (per the robobun status comment) specified the single-pass requirement, so the design direction has some prior agreement, but the implementation itself has not been human-reviewed.

Other factors

• My two prior inline comments were both resolved: the Proxy-of-array replacer classification now uses spec IsArray (with a test covering plain Proxy, trapped Proxy, and revoked Proxy), and the dead getPropertyValue cleanup is deliberately left to #39618 to avoid a merge conflict.
• Test coverage is thorough: the JSON5 suite compares call-by-call with JSON.stringify (keys, holders, order), and includes a GC-stress test (Bun.gc(true) inside the replacer), a stack-overflow test, cycle-breaking, boxed primitives, holes, and the Proxy edge case. YAML and TOML tests cover their format-specific interactions (anchors under replacer, TOML layout following replaced values, Date/Temporal through the replacer).
• All comment-cop and CodeRabbit comments on the timeline are marked resolved.
• The write_inline fallthrough to err_in_array in TOML looked suspicious at first (a scalar that is none of bool/number/bigint/string/date/temporal), but read only produces Value::Scalar for values that pass those type checks or for the null/undefined/symbol/function-in-array error path, so it is a defensive dead arm rather than a reachable bug.

[claude]

I reviewed this PR and didn't find any bugs. Because it substantially restructures the YAML and TOML stringifiers (single-read into heap-allocated Node/Value records, new anchor/alias bookkeeping) and stores JSValues on the Rust heap across replacer calls, a human look at the GC-rooting and the YAML alias-vs-function-replacer semantics would still be worthwhile.

What was reviewed:

• Replacer::from_js key-list handling (spec IsArray via JSC::isArray, string/number filtering, dedup) against ECMA-262 §25.5.2 step 4.b.
• MarkedArgumentBuffer rooting of scalars/collections in YAMLObject::read and TOMLObject::read while replacers/getters run JS.
• The two prior inline findings (dead getPropertyValue, Proxy-of-array replacer) — both addressed by the author.

Extended reasoning...

Overview

This PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify and Bun.JSON5.stringify, matching JSON.stringify's replacer protocol (function or property-list array). It adds a shared src/runtime/api/stringify_replacer.rs (Replacer and Properties), two new C++ bindings (getMayBeIndex, isArrayIncludingProxy), and substantially rewrites the YAML and TOML stringifiers from their previous two-pass designs into a single depth-first read into native Node/Value records followed by a JS-free write pass. Docs, .d.ts, type fixtures, and ~400 lines of tests across three test files are included.

Security risks

None identified. The replacer runs user JS during the read pass, which the PR handles by rooting every stored JSValue in a MarkedArgumentBuffer and by re-establishing no raw pointers across the calls. Input is user-controlled JS values, not external bytes; the existing stack-overflow guards and cycle detection are preserved on both the read and write recursions.

Level of scrutiny

High. This is not a mechanical change: the YAML stringifier's anchor/alias machinery is entirely reworked (known_collections now maps to NodeId, anchored is gone, function-replacer mode changes shared-object output from anchor+alias to per-occurrence), and TOML's Layout classification is replaced by an eager Value tree. The GC-safety story is subtle — JSValues live in Vec<Item>/Vec<Value> on the Rust heap while a replacer or getter can trigger GC, so correctness depends on every such value also being appended to roots. I traced the append sites (strings and objects in YAML read, scalars in TOML read) and they cover every Item::Scalar/Value::Scalar that reaches the write pass; primitives (null/number/bool) don't need rooting.

Other factors

The test coverage is thorough (call-for-call comparison with JSON.stringify, GC stress, stack-overflow, Proxy replacers, shared/cyclic objects, format-specific interactions with Date/Temporal and TOML layout). Both of my prior inline findings were addressed (Proxy-of-array now uses spec IsArray with a test; the dead getPropertyValue is intentionally left for #39618 to remove, per the author's explanation). All comment-cop and CodeRabbit threads are resolved. However, the API-design decision that a function replacer disables YAML aliasing for non-cyclic shared objects, and the YAML.stringify return-type change to string | undefined, are user-visible behavior changes that a maintainer should sign off on.

[coderabbitai]

Note

GitHub couldn't provide a complete incremental comparison for this pull request, so CodeRabbit is performing a full review instead. This review may take a little longer.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@docs/runtime/json5.mdx`:
- Around line 154-156: Document that when the replacer returns undefined for the
root key "" , JSON5.stringify, TOML.stringify, and YAML.stringify return
undefined. Update docs/runtime/json5.mdx (154-156), docs/runtime/toml.mdx
(157-164), and docs/runtime/yaml.mdx (141-143), plus the corresponding `@returns`
descriptions in packages/bun-types/bun.d.ts (1101-1114, 1530-1545, and
2129-2141); no other behavior or documentation needs changing.

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ℹ️ Review info

⚙️ Run configuration

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Review profile: ASSERTIVE

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Run ID: 39c97365-116c-434e-8aeb-f22d6d751283

📥 Commits

Reviewing files that changed from the base of the PR and between f2fe7d3 and 5f7da35.

📒 Files selected for processing (17)

• docs/runtime/json5.mdx
• docs/runtime/toml.mdx
• docs/runtime/yaml.mdx
• packages/bun-types/bun.d.ts
• src/jsc/JSValue.rs
• src/jsc/bindings/bindings.cpp
• src/runtime/api.rs
• src/runtime/api/JSON5Object.rs
• src/runtime/api/TOMLObject.rs
• src/runtime/api/YAMLObject.rs
• src/runtime/api/stringify_replacer.rs
• test/integration/bun-types/fixture/json5.ts
• test/integration/bun-types/fixture/toml.ts
• test/integration/bun-types/fixture/yaml.ts
• test/js/bun/json5/json5.test.ts
• test/js/bun/toml/toml.test.ts
• test/js/bun/yaml/yaml.test.ts

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[claude]

I reviewed this PR and the automated bug hunt found no issues. Because it adds user-facing API surface and restructures the TOML and YAML stringifiers into a read-then-write design with heap-held JSValues across replacer callbacks, a human look is still worthwhile.

• Checked GC rooting: every Value::Scalar/Item::Scalar and collection key stored in a Vec is also appended to the MarkedArgumentBuffer before any further JS runs.
• Checked the YAML anchor semantics change (function replacer → per-occurrence write, cycle still anchored) against the tests and docs; the known_collections.remove after descent is what keeps cycles anchored while re-visiting siblings.
• Checked Replacer::from_js against ECMA-262 §25.5.2 step 4.b (IsArray via JSC::isArray, string/number/boxed entries only, dedup) — the earlier Proxy-of-array gap is fixed and covered by a test.
• Checked exception propagation on the new C++ bindings and the ? paths through replace_* / Properties::next.

Extended reasoning...

Overview

This PR implements the replacer argument (function or property-list) for Bun.YAML.stringify, Bun.TOML.stringify and Bun.JSON5.stringify, matching JSON.stringify's protocol minus toJSON. It adds a shared stringify_replacer.rs module (Replacer + Properties), two small C++ bindings (getMayBeIndex, isArrayIncludingProxy), and restructures the TOML and YAML stringifiers from a two-pass JS-reading design into a single read pass that materializes native Value/Node records, followed by a JS-free write pass. JSON5 stays single-pass with the replacer inlined. Docs, .d.ts, type fixtures and ~350 lines of tests across the three suites are updated.

Security risks

None identified. The replacer is a user callback invoked on user data; the input surface is unchanged. No filesystem, network, auth, or privilege boundaries are touched. The GC-safety surface (JSValues in Rust Vecs while user JS runs) is handled with a MarkedArgumentBuffer.

Level of scrutiny

High. This is not a mechanical change: it adds public API, changes observable YAML behavior (getters read once; shared objects under a function replacer are written per-key instead of anchored), and rewrites two stringifiers around a new intermediate representation with heap-held JSValues live across arbitrary user callbacks. The GC-rooting story is careful and tested (including a Bun.gc(true)-in-replacer test), but the combination of native memory safety, JSC exception-scope discipline, and an API contract that promises JSON.stringify parity is exactly the class of change REVIEW.md flags for maintainer sign-off.

Other factors

The PR was requested by a maintainer and has already been through several rounds of bot feedback (comment-cop, CodeRabbit, two prior nits from this reviewer — all resolved). Test coverage is thorough, including call-order comparison against JSON.stringify, Proxy-of-array replacers, GC stress, stack-overflow guards, and format-specific interactions (Temporal in TOML, anchors in YAML). Still, the YAML anchor/alias semantics change and the read-into-records architecture are design decisions a maintainer should confirm.

[claude]

I reviewed this PR and the bug hunting system found no issues. Both earlier findings (the Proxy-of-array replacer classification and the getPropertyValue overlap with #39618) have been addressed. Given the scope — new user-facing API surface across three stringifiers, a read-into-native-records rewrite of the YAML and TOML write paths, GC rooting via MarkedArgumentBuffer for heap-stored JSValues, and the design choice that a function replacer suppresses YAML anchors for non-cyclic shared objects — a maintainer look is still worthwhile.

What was reviewed:

• GC safety of Value::Scalar / Item::Scalar and known_collections keys — every stored JSValue is appended to roots before user JS runs.
• Replacer::from_js now uses spec IsArray (is_array_including_proxy); revoked-Proxy TypeError propagates.
• Exception handling in the new getMayBeIndex / isArrayIncludingProxy bindings — both under a ThrowScope with RETURN_IF_EXCEPTION / check_slow.
• YAML block-style output: the prop_value_needs_newline → !matches!(item, Item::Scalar(_)) change preserves the same set of values that get a leading newline.

Extended reasoning...

Overview

This PR implements the replacer argument for Bun.YAML.stringify, Bun.TOML.stringify and Bun.JSON5.stringify, matching JSON.stringify's protocol (function replacer with holder-as-this, or a property-name array). It adds a shared stringify_replacer.rs module with Replacer and Properties, two new C++ bindings (getMayBeIndex, isArrayIncludingProxy) with Rust wrappers, and rewrites the TOML and YAML stringifiers from a JS-driven walk into a read-once-into-native-records / write-from-records design so a function replacer runs exactly once per property. Docs, .d.ts, type fixtures and ~350 lines of new tests (including a call-by-call comparison against JSON.stringify) are updated. Roughly 17 files, ~1200 lines of diff.

Security risks

None identified. The replacer is user-supplied JS invoked with the same values JSON.stringify would pass; no new privilege boundary. The getMayBeIndex binding is a plain [[Get]] under a ThrowScope. Prototype pollution is not in play (the stringifiers read, they do not write into user objects; the root holder is a fresh { "": value }).

Level of scrutiny

High. This is not a mechanical change: TOML and YAML stringify are structurally rewritten, JSValues are now stored on the Rust heap across calls that run user JS (rooted via a MarkedArgumentBuffer), and there is a user-visible API/behavior addition with a deliberate design decision (with a function replacer, YAML writes a shared non-cyclic object once per key instead of once with an anchor and aliases; without a replacer, aliasing is unchanged). The YAML.stringify return type changes to string | undefined. These are exactly the categories the approval guidelines flag for human review.

Other factors

The two nits I raised on earlier revisions were both handled — the is_array → spec-IsArray fix landed with a covering test, and the getPropertyValue cleanup was intentionally deferred to #39618 to avoid a merge conflict. The comment-cop and CodeRabbit threads are all resolved. Test coverage is thorough (call order and holder identity checked against JSON.stringify, GC-under-replacer test, Proxy/revoked-Proxy, deep-value RangeError, single-getter-read assertions in all three formats, TOML layout following replaced values, YAML anchor behavior with and without a function replacer). Given the size, the GC-sensitive native changes and the API-surface addition, deferring to a maintainer is the right call rather than auto-approving.