[robobun]

Problem

• console.log, Bun.inspect, an uncaught error and console.table never return when a value's iterator never reports done. An uncaught Error that carries a Map prints forever at 100% CPU once Map.prototype[Symbol.iterator] is an endless generator. console.table runs until the OOM kill.
• The native formatter walks these values with JSC::forEachInIterable (bindings.cpp:4256), which ends only on done. Callers include print_map_like and TablePrinter::print_table.

Fix

• Bun__ConsoleObject__forEachLimited (src/jsc/bindings/ConsoleObject.cpp) is forEachInIterable with the iterator protocol bounded. A collection gives a replaced iterator as many steps as it reports elements: size for a Map or a Set, the length for an array or typed array. An iterable with no element count gets a budget: 1000 rows in console.table, 100 members of AggregateError.errors.
• A walk that stops with elements left closes the iterator, like break in a for-of loop, and the output ends with ... more items, ... more rows or ... more errors.
• Output that ends today does not change, with one exception: an iterator that yields more than size is cut there and marked. Node cuts there too.
• Verified: test/js/bun/util/inspect.test.js and test/js/bun/console/console-table.test.ts. bun 1.4.3 fails 16 of 24 new tests (8 are controls). Also ran the console and inspect suites.

Background

• These four entry points share one native formatter.
• forEachInIterable reads an unmodified Array, Map or Set from its own storage, and this PR keeps that. Once user code can observe the iteration, user code decides when the walk ends.
• Node bounds the Map walk with min(maxArrayLength, map.size). 1000 rows is Chrome's console.table limit (InjectedScript::wrapTable in V8).

Notes

Reworked after a self-review

The first version of this PR also stopped every Map and Set preview at 100 entries, and read a Map or a Set from its storage even when user code had replaced the iterator. Both are gone.

• The 100-entry limit is a change of default output with no opt-out. console.dir(map, { maxArrayLength: null }) prints every entry today. That limit belongs with maxArrayLength support: console.dir: honor maxArrayLength in the native formatter #31809, Bun.inspect() with depth: Infinity truncates arrays #15193, and console: cap Map/Set entries at 100 in the native formatter #35826 which proposed the same limit.
• The unconditional storage read printed Map(2) {} for a Map subclass that keeps its entries outside the inherited storage. quick-lru has that shape: it never calls super.set. The test a Map subclass that keeps its entries elsewhere prints them pins the output for Bun.inspect and Bun.inspect.table.

Output, before and after

Map.prototype[Symbol.iterator] = function* () { for (;;) yield ["k", "v"]; };
console.log(new Map([["a", 1], ["b", 2]]));
// bun 1.4.3:  Map(2) { "k": "v", "k": "v", ... }          never ends
// node 26:    Map(2) { 'k' => 'v', 'k' => 'v' }
// this PR:    Map(2) { "k": "v", "k": "v", ... more items }

A Map, Set, Map iterator or Set iterator that user code did not touch prints exactly as before, at any size. A script that prints 16 shapes (Proxy of a Map, a Map from node:vm, nested Maps and Sets, all four iterator kinds, Headers, URLSearchParams, FormData, errors, AggregateError) in both modes, plus 8 tables, gives byte-identical output on bun 1.4.3 and this branch. The one difference is a Map subclass whose iterator yields 2 entries while size says 1: it now prints 1 entry and ... more items.

Bounds, by kind of value

• Array in fast mode, Map or Set whose iteration user code cannot observe: read from storage, as forEachInIterable does. No bound is needed.
• Map or Set otherwise (a subclass, an own Symbol.iterator, a replaced prototype method): the size property, read once. A finite size is taken at its word, so a subclass that keeps 1500 entries outside the inherited storage prints all of them. A size that is not a finite count (Infinity, NaN, negative) falls back to the budget. A subclass that reports a huge finite size and never stops yielding is not bounded here. Node bounds it with maxArrayLength, and the display limit in console: cap Map/Set entries at 100 in the native formatter #35826 will do the same.
• Array or typed array otherwise: its length.
• Map iterator or Set iterator: the size of the collection it walks. A replaced next cannot yield more than that.
• Anything else (a generator, Headers, a WeakMap that user code made iterable): the caller's budget.

console.table

• Node's console.table uses Object.keys for anything that is not a Map or a Set, so it never calls Symbol.iterator on a plain object. bun reads generators and Headers through the iterator, and console-table.test.ts covers that (a generator is not consumed twice, headers object), so this PR keeps it and adds the budget.
• The test an array, a typed array, a Map and a Set print every row pins 1500 rows for each.
• One step past the bound tells a cut table from one that ends exactly there. A generator of 1000 rows prints no marker.

Exceptions and iterator close

• A throw from next, from the callback, or from return() propagates as before.
• The new tests pass under BUN_JSC_validateExceptionChecks=1. A script of 16 error paths prints identical results on bun 1.4.3 and this branch.

Same class, not fixed here

These sites also run a user iterator without a bound. They are in other subsystems and are handed off as separate work.

• util.inspect(map.entries()): previewEntries in src/js/internal/util/inspect.js uses Array.from. An earlier revision of this description said util.inspect is already bounded. That holds for a Map or a Set, not for their iterators.
• new Console().table(map): src/js/builtins/ConsoleObject.ts.
• expect(iterable).toBeEmpty(): src/runtime/test_runner/expect/toBeEmpty.rs walks the whole iterable.
• The expect() diff and snapshot printer: src/runtime/test_runner/pretty_format.rs. A snapshot must list every entry, and a storage read would rewrite stored snapshots of Map subclasses, so it needs the size-bounded protocol walk, not a display limit.

Other changes

• JSValue::for_each_with_context is removed. console.table was its last caller.
• bun still advances a Map or Set iterator when it prints it. Node does not. That is unchanged here.

Related PRs

• console: cap Map/Set entries at 100 in the native formatter #35826: the 100-entry limit for Map and Set. It was closed in favour of an earlier revision of this PR that carried the limit. The limit is no longer here.
• console.dir: honor maxArrayLength in the native formatter #31809 and Bun.inspect() with depth: Infinity truncates arrays #15193: maxArrayLength support in the native formatter.
• console.table: walk an array by its own properties instead of trusting length #41119 moves arrays off the iterator path in console.table. It keeps the iterator path for generators, Maps and Sets.

Origin

Two rows of a fuzzing ledger of hang-class bugs on main: an uncaught Error that carries a Map with a replaced iterator, and console.table of an object with an endless Symbol.iterator.

Suites run on the debug build

test/js/bun/util/inspect.test.js, inspect-error.test.js, test/js/bun/console/, test/js/web/console/ (230 pass), test/js/node/util/node-inspect-tests/ and util.test.js (228 pass). util-inspect-long-running.test.mjs took 5.5 s against the 5 s default in my CPU-capped debug container (245 ms on a release build). It formats a plain object with the JS util.inspect and calls none of the changed code.

---

no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/util/inspect.test.js

[robobun]

Status

Reproduced on bun 1.4.3 and on a debug build of main:

# prints `"k": "v",` forever at 100% CPU
bun -e 'Map.prototype[Symbol.iterator] = function* () { for (;;) yield ["k", "v"]; }; const e = new Error("boom"); e.ctx = new Map([["a", 1]]); throw e'

# never returns, RSS grows until the OOM kill
bun -e 'console.table({ host: "db1", *[Symbol.iterator]() { for (;;) yield { t: 1 }; } })'

With this branch the first prints ctx: Map(1) { "k": "v", ... more items } and exits with code 1, which is what node prints plus the marker. The second prints 1000 rows and ... more rows, then exits with code 0.

Fail-before: USE_SYSTEM_BUN=1 bun test test/js/bun/util/inspect.test.js test/js/bun/console/console-table.test.ts -t "bounds a replaced|never ends|bounded number of rows" gives 16 fail, 8 pass (the 8 are controls for output that must not change). bun bd test on the same files gives 24 pass.

Self-reviewed: the review asked for a narrower shape, and the branch now has it. The 100-entry display limit for Map and Set is out (it belongs with maxArrayLength, see #35826 and #31809). A Map or a Set is read from its storage only when user code cannot observe the iteration, so a Map subclass that keeps its entries elsewhere prints as before. The same class of bug in util.inspect(map.entries()), new Console().table, toBeEmpty and the expect() diff printer is named in the PR notes and handed off as separate work.

PR: #42264

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Walkthrough

The changes add bounded traversal for console tables, collection previews, and AggregateError output. They apply row and entry limits, render truncation markers, close general iterators, and add regression tests for infinite and replaced iterators.

Changes

Bounded collection inspection

Layer / File(s)	Summary
Bounded traversal helper src/jsc/ConsoleObject.rs, src/jsc/JSValue.rs, src/jsc/bindings/ConsoleObject.cpp	Adds limited traversal across Rust and native code. The helper reports truncation, propagates callback errors, and closes general iterators.
Bounded table and preview output src/jsc/ConsoleObject.rs, src/jsc/VirtualMachine.rs	Limits table rows and collection previews to defined budgets. Map and Set storage traversal remains direct. Truncation markers are rendered for tables, previews, and AggregateError output.
Limit and iterator regression coverage test/js/bun/console/console-table.test.ts, test/js/bun/util/inspect.test.js	Tests limits, markers, exact-budget behavior, replaced iterators, non-terminating iterators, collection formatting, iterator closure, and AggregateError inspection.

Suggested reviewers: dylan-conway

Priority: ➖ Normal

Merge Risk: 🟡 Moderate · up to b2ea8

Formatting a Map or Set that triggers an exception during rendering may continue traversing entries without a bound, which can hang console output. This behavior should be resolved before merge.

🚥 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 clearly explains the problem, implementation, behavior, verification, scope, and related limitations. It does not use the exact template headings, but it provides the required change s…
Title check	✅ Passed	The title is concise and accurately identifies the main change: preventing console formatting from hanging on non-terminating iterators.

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In `@src/jsc/bindings/ConsoleObject.cpp`:
- Around line 149-157: Update the visitStorageEntry callback to return
WTF::IterationStatus::Done immediately after invoking callback when
scope.exception() is set; otherwise preserve the existing Continue behavior and
limit handling so storage traversal stops after the first callback error.

In `@src/jsc/ConsoleObject.rs`:
- Around line 1010-1016: Cap the array-like traversal limit in the
limit-selection logic using a practical MAX_ROWS_FROM_ARRAY_LIKE ceiling instead
of allowing u32::MAX. Define or reuse MAX_ROWS_FROM_ARRAY_LIKE near the existing
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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[robobun]

^{Updated 8:41 AM PT - Sep 11th, 2026}

✅ @robobun, your commit b2ea8fdbd13596a71cffd0e37b4615a920be97bf passed in Build #114319! 🎉

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⚠️ Outside diff range comments (2)

src/jsc/bindings/ConsoleObject.cpp (1)

149-157: 🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Stop the Map/Set storage walk after a formatter exception.

visitStorageEntry always returns Continue after the formatter callback. Unlike the generic iterator path, it does not check scope.exception(). A reachable console.table or custom inspect callback can leave a pending exception, while ctx.err only suppresses later formatting. With a u32::MAX Map/Set limit, the storage walk can continue through the remaining entries before returning the exception. Check scope.exception() immediately after callback and return WTF::IterationStatus::Done when set.

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In `@src/jsc/bindings/ConsoleObject.cpp` around lines 149 - 157, Update the
visitStorageEntry callback to check scope.exception() immediately after
callback; return WTF::IterationStatus::Done when an exception is pending,
otherwise preserve the existing Continue behavior so Map/Set storage iteration
stops promptly on formatter errors.

src/jsc/ConsoleObject.rs (1)

1010-1016: 🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Use the 1,000-row budget for array-like iterables. TablePrinter::print_table converts an array's reported length to u32, while Bun__ConsoleObject__forEachLimited traverses arrays through their replaceable Symbol.iterator. An array with length = 0xffffffff and a non-terminating iterator can therefore process up to u32::MAX rows, causing prolonged CPU and memory exhaustion. Apply the length-based limit only to Map and Set values handled through trusted storage.

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In `@src/jsc/ConsoleObject.rs` around lines 1010 - 1016, Update the row-limiting
logic in TablePrinter::print_table and Bun__ConsoleObject__forEachLimited so
length-based iteration limits are applied only to Map and Set values using
trusted storage; do not use an array-like value’s reported length or replaceable
Symbol.iterator for the budget. Ensure array-like iterables remain capped at
1,000 rows even when their length is u32::MAX or their iterator does not
terminate.

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Outside diff comments:
In `@src/jsc/bindings/ConsoleObject.cpp`:
- Around line 149-157: Update the visitStorageEntry callback to check
scope.exception() immediately after callback; return WTF::IterationStatus::Done
when an exception is pending, otherwise preserve the existing Continue behavior
so Map/Set storage iteration stops promptly on formatter errors.

In `@src/jsc/ConsoleObject.rs`:
- Around line 1010-1016: Update the row-limiting logic in
TablePrinter::print_table and Bun__ConsoleObject__forEachLimited so length-based
iteration limits are applied only to Map and Set values using trusted storage;
do not use an array-like value’s reported length or replaceable Symbol.iterator
for the budget. Ensure array-like iterables remain capped at 1,000 rows even
when their length is u32::MAX or their iterator does not terminate.

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[robobun]

@coderabbitai the two "outside diff range" items in the review of 7ad796e repeat the two findings that were withdrawn earlier on this PR. Neither needs a change.

1. visitStorageEntry returns Continue after a callback throws (thread). forEachInMapStorage and forEachInSetStorage check scope.exception() right after the callback and return before they read the IterationStatus, so the walk stops at the first error. A 3-entry Map whose values throw from Bun.inspect.custom calls the hook once.
2. Array-like rows bounded by length (thread). The limit is the real element count, read from the internal length. A replaced iterator gets no more steps than the builtin iterator takes for the same array, so new Array(0xffffffff) is no worse with a replaced iterator than without one. That sparse-array cost exists on main and console.table: walk an array by its own properties instead of trusting length #41119 covers it. A 1000-row budget for arrays would cut real arrays that print in full today and in node. The test an array, a typed array, a Map and a Set print every row pins that.

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@robobun Thanks for the clarification. The two findings do not require changes.

forEachInMapStorage and forEachInSetStorage stop after the callback sets scope.exception(). The Continue return value is not used in that path.

For array-like values, the actual internal length is the correct console.table traversal limit. This preserves full output for arrays and typed arrays. The sparse-array behavior is separate from this change and remains tracked in #41119.

---

🧠 Learnings used

Learnt from: robobun
Repo: oven-sh/bun PR: 42264
File: src/jsc/ConsoleObject.rs:1010-1016
Timestamp: 2026-09-11T02:28:41.188Z
Learning: In `src/jsc/ConsoleObject.rs`, `TablePrinter::print_table` must preserve full-row `console.table` output for arrays and typed arrays. For array-like iterable values, the traversal limit is the value's actual length, so a replaced `Symbol.iterator` cannot yield more rows than that length. `ArrayBuffer` is array-like but non-iterable, so it does not enter this traversal path. Large sparse-array `console.table` output is a separate concern tracked by GitHub issue `#41119`.

---

Learnt from: robobun
Repo: oven-sh/bun PR: 42264
File: src/jsc/bindings/ConsoleObject.cpp:149-157
Timestamp: 2026-09-11T02:28:36.112Z
Learning: In `src/jsc/bindings/ConsoleObject.cpp`, callbacks passed to JavaScriptCore `JSC::forEachInMapStorage` and `JSC::forEachInSetStorage` do not need to return `WTF::IterationStatus::Done` after setting a JavaScript exception. The JavaScriptCore `IteratorOperations.h` traversal checks `scope.exception()` immediately after each callback, invokes its exception handler, and stops before it reads the callback’s `IterationStatus`.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

Pushed cf82ac4 on top of this branch. It adds two test cases from #35826 and changes no source file. #35826 has the same Map and Set limit as this PR, so it is closed in favor of this one.

• the preview of a large Map stays small: a 10,000-entry Map prints 103 lines and ends with ... 9900 more items.
• the limit counts stored entries, not what a size getter reports: a Map subclass with get size() { return 1 } and 500 entries still stops at 100 entries.

The second case differs from #35826 by design. #35826 walks every entry, so it prints ... 400 more items. This PR stops the walk at the limit, so size is the only source for the count. When size is below the number of entries printed, the marker reads ... more items.

Both cases fail on bun 1.4.3 (10,002 and 502 lines). On the debug build of this branch, test/js/bun/util/inspect.test.js gives 95 pass and test/js/bun/console/console-table.test.ts gives 41 pass. The other five cases from #35826 already had an equivalent here. The counts in the PR body are updated (20 of 23 new tests fail on bun 1.4.3).

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⚠️ Outside diff range comments (1)

src/jsc/ConsoleObject.rs (1)

1010-1016: 🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Cap array iterator traversal at MAX_ROWS_FROM_ITERATOR. The has_element_count branch passes Array.length to for_each_limited, while Bun__ConsoleObject__forEachLimited obtains the replaceable iterator with JSC::iteratorForIterable. An array with length === 0xffffffff and an endless iterator can therefore collect billions of rows and exhaust memory. Use the smaller of the array length and MAX_ROWS_FROM_ITERATOR for this path.

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In `@src/jsc/ConsoleObject.rs` around lines 1010 - 1016, Update the
has_element_count path before calling for_each_limited to pass the smaller of
the array length and MAX_ROWS_FROM_ITERATOR, while preserving the existing
iterator handling and truncation behavior.

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In `@src/jsc/ConsoleObject.rs`:
- Around line 1010-1016: Update the has_element_count path before calling
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MAX_ROWS_FROM_ITERATOR, while preserving the existing iterator handling and
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Code review found no issues

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

This pull request has been reviewed before and this review found new issues. Where they share a root cause, one fix may close them together.

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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

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⚠️ Outside diff range comments (1)

src/jsc/ConsoleObject.rs (1)

4633-4633: 🎯 Functional Correctness | 🟠 Major | 🏗️ Heavy lift

Preserve non-finite size values until bound selection.

print_map_like() and print_set() convert size with coerce_to_i32(). NaN and Infinity become zero, so the zero-length branch skips iteration and renders a productive Map or Set subclass as empty.

Bun__ConsoleObject__forEachLimited already keeps the 1,000-entry budget for non-finite sizes when it receives -1. Keep the raw size separate from the display count. For non-finite sizes, skip the zero-length return and pass None so the native helper rereads the value. Add Map and Set regressions.

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In `@src/jsc/ConsoleObject.rs` at line 4633, Update print_map_like() and
print_set() to retain the raw size alongside the coerced display count; detect
non-finite size values before the zero-length early return and pass None to
Bun__ConsoleObject__forEachLimited so it rereads the size and preserves the
1,000-entry budget. Add regression coverage for productive Map and Set
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In `@src/jsc/ConsoleObject.rs`:
- Line 4633: Update print_map_like() and print_set() to retain the raw size
alongside the coerced display count; detect non-finite size values before the
zero-length early return and pass None to Bun__ConsoleObject__forEachLimited so
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[robobun]

@coderabbitai on the "outside diff range" item in the review of b2ea8fd (coerce_to_i32 turns a size of NaN or Infinity into 0, so print_map_like and print_set print Map {} and skip the walk): that is how main behaves today, and this PR does not change it. It ends at once, so it is not part of the hang this PR fixes. The coercion of size for the header is the subject of #32279. Changing what a Map that reports Infinity prints belongs there, not in this change. console.table has no such early return, and the test a Map subclass whose size is not a finite count covers that path.

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@robobun Thanks for the clarification. The finding does not apply to this PR.

coerce_to_i32 handling of NaN and Infinity is existing behavior. print_map_like and print_set stop without an unbounded walk, so this behavior does not affect the hang fix.

#32279 tracks the separate header-size coercion behavior. The console.table path remains covered by a Map subclass whose size is not a finite count.

---

✏️ Learnings added

Learnt from: robobun
URL: https://github.com/oven-sh/bun/pull/42264

Timestamp: 2026-09-11T15:25:58.908Z
Learning: In `src/jsc/ConsoleObject.rs`, existing `coerce_to_i32` behavior converts a Map or Set `size` of `NaN` or `Infinity` to `0`, causing `print_map_like` and `print_set` to print an empty collection and skip traversal. This terminates formatting and is outside the unbounded-iteration fix in PR `#42264`. GitHub issue `#32279` tracks the separate behavior for coercing `size` in collection headers. `console.table` does not take this early-return path and has separate coverage for non-finite Map subclass sizes.

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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

A data point for the size bound, from the review of #42359 (the JS Console#table twin of this change).

The Elsewhere fixture here is labelled as the quick-lru shape, but it reports an honest size. The real package does not. quick-lru 7.3.0 caps size at maxSize, and its iterator also yields the older generation:

const lru = new QuickLRU({ maxSize: 3 });
for (let i = 0; i < 5; i++) lru.set("k" + i, i);
lru.size;        // 3
[...lru].length; // 5

• console.log(lru): Node cuts at size too (formatMap uses min(maxArrayLength, size)), so the bound matches Node there.
• console.table(lru): Node and bun 1.4.2 print 5 rows. A walk that stops at size prints 3 rows and the marker.

#42359 first had the same bound in new Console().table(map) and dropped it for this reason. It leaves that loop as Node has it. If the table family keeps the bound here, the two paths differ for this package, so the rule is worth one decision for both.

[robobun]

#42359 now edits the same four native call sites as this PR: the two value.for_each calls in print_map_iterator_like, the table data walk in TablePrinter::print_table, and the AggregateError.errors walk in VirtualMachine.rs.

The two changes answer different questions and fit together.

• This PR bounds a walk that runs the iterator protocol.
• console, Bun.inspect, util.inspect: print a Map or Set iterator without running it #42359 stops running the protocol for one kind of value: a Map or Set iterator is read from its collection through Bun__inspect__forEachRemainingEntry (src/jsc/bindings/UtilInspect.cpp). The print no longer uses up the caller's iterator, and a replaced next is not called, so that walk ends by construction. Every other value still goes to JSValue::for_each.

The PR that lands second has a textual conflict at those sites. If this one lands first, the dispatcher of #42359 (for_each_listed) becomes a two-type test at the top of forEachLimited. If #42359 lands first, the row "Map iterator or Set iterator: the size of the collection it walks" in this description has no caller left and can go.