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Node coerces start, then end, returns "" for an empty range, and only
then resolves the encoding. Bun parsed the encoding first, so
buf.toString("bogus", 1, 1) threw ERR_UNKNOWN_ENCODING where Node
returns "". A zero-length buffer now takes the same path, because Node
has no early return for it.
Node compares start and end with the length before it truncates them,
and a comparison accepts a BigInt. A BigInt start that is <= 0 or
>= length and a BigInt end that is > length now behave as in Node.
Every other BigInt still throws a TypeError, as Node's MathTrunc does.
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Status How I reproduced it, on canary 1.4.3 (b52d513) against Node v26.3.0: Buffer.from("abc").toString("bogus", 1, 1); // Node: "". Bun: throws ERR_UNKNOWN_ENCODING
Buffer.from("abc").toString("utf8", 0n); // Node: "abc". Bun: throws TypeError
Review: all threads have a reply and are resolved. Two findings are declined with evidence in the thread (repeated coercion of PR: #43432 |
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ChangesBuffer string argument handling
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Actionable comments posted: 3
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@src/jsc/bindings/JSBuffer.cpp`:
- Around line 2307-2329: Update the range handling around the start/end coercion
logic to operate on the original arg2 and arg3 values for each comparison and
subsequent numeric truncation, preserving Node’s repeated Symbol.toPrimitive
calls instead of reusing cached startValue or endValue primitives. Keep the
existing BigInt behavior and add stateful Symbol.toPrimitive coverage for both
arguments.
- Line 2343: Update the Buffer.prototype.toString encoding path around
parseEncoding so the supplied encoding argument is coerced with the default hint
before parsing, matching the required JavaScript behavior. Add a regression test
using an object that returns different values for default and string coercion,
verifying the default-hint result is used for a non-empty buffer.
In `@test/js/node/buffer.test.js`:
- Line 5055: Update the Bun subprocess command in the test around Bun.spawn so
the standalone buffer-tostring-args-fixture.js source runs via Bun’s -e option,
using readFileSync(fixture, "utf8") as the evaluated source; keep the Node
command unchanged as [exe, fixture].
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Updated 5:20 AM PT - Sep 19th, 2026
✅ @robobun, your commit 74401d7c32d89239529d283246999a12f1844e6a passed in 🧪 To try this PR locally: bunx bun-pr 43432That installs a local version of the PR into your bun-43432 --bun |
Each comment in jsBufferPrototypeFunction_toStringBody is now one line. The tests assert that Bun coerces start and end once each. Node's JS coerces start up to three times and end twice. The fixture prints with console.log, which costs about one second less in a debug build than process.stdout.write.
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The follow-up push collapses the comments to one line each and now pins the single coercion per argument in the tests, which covers what I raised last time; I re-read the updated diff and found no bugs, but since this changes user-observable Buffer.prototype.toString semantics (BigInt start/end now accepted, encoding no longer validated for an empty range) and locks in a deliberate divergence from Node's coercion count, a maintainer look at that behavioral decision is still worthwhile.
What was reviewed:
- Every user-JS entry in the new path (
toPrimitive,toNumber,toLength,parseEncoding) is followed byRETURN_IF_EXCEPTIONbefore its result is used;JSBigInt::compare(JSValue, int64_t/uint64_t)overloads are the same onesBunIDLConvertNumbers.halready uses. - NaN, ±Infinity, negatives, and out-of-range doubles are settled by the
!(fstart > 0)/fstart >= byteLengthcompares before thesize_tcast;endgoes throughtoLengthso a negative or NaN end lands at 0 and hits theend <= startreturn. - Traced the Node v26
toStringbranches (start <= 0,start >= len,MathTrunc || 0,end > len) against the new code for null, numeric strings, fractional, Infinity, and BigInt inputs — results agree, and the differential fixture isskipIf(!nodeExe())with concurrent pipe draining.
Extended reasoning...
Overview
Two commits rework jsBufferPrototypeFunction_toStringBody in src/jsc/bindings/JSBuffer.cpp (about 30 changed lines) so the argument order matches Node's lib/buffer.js toString: start is coerced first, then end, the empty-range return fires, and only then is the encoding parsed. The zero-length early return is removed, and BigInt start/end values are accepted where Node's comparisons would settle them without reaching MathTrunc. Tests add a describe block in test/js/node/buffer.test.js plus a differential fixture test/js/node/buffer-tostring-args-fixture.js that runs under both Node and Bun. The second commit (983989c) responded to the prior review: comments are now one line each and the logged helper pushes on every coercion so the test asserts exactly one coercion per argument.
Security risks
None specific. The function reads user-controlled numbers and coerces them, but every conversion is bounded before the size_t cast, and jsBufferToString clamps against the live byteLength so a resize() inside a coercion hook cannot produce an out-of-bounds read (I noted the Node-vs-Bun divergence there in the prior review; it is pre-existing and unchanged by this PR).
Level of scrutiny
Moderate. This is native code that runs user JS mid-function, so the exception-check and detach-safety rules apply; I verified each RETURN_IF_EXCEPTION is present and that the JSBigInt::compare overloads exist in-tree. The logic itself is small and the test matrix (2 buffers x 5 encodings x 14 starts x 12 ends plus coercion-order rows) is thorough. What keeps this from a plain approve is that it is a user-visible behavioral change to a core Node-compat API and it deliberately codifies a divergence (one coercion per argument versus Node's up to three) — that trade-off is a maintainer decision, not something an automated review should sign off alone.
Other factors
No CODEOWNERS entry covers the changed files. The earlier inline notes about non-8-bit receivers, resizable-ArrayBuffer clamping, and the encoding coercion hint remain as pre-existing divergences that the author chose not to fold in here; none blocks this change. The bug hunt exited on dry_streak with no findings, and the new commit addressed the only nit from the previous run.
Still open from earlier reviews (2):
- Unresolved: 2 minor or pre-existing.
Move the note about the order of the encoding next to the encoding parse, so that no two comment lines are adjacent.
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I re-reviewed after the two follow-up commits and found no bugs; the comment trimming addresses my earlier nit. Since this changes the observable semantics of Buffer.prototype.toString in native code (removes the zero-length early return, flips the >/>= boundaries, adds BigInt paths), a human look is still worthwhile.
What was reviewed:
- Traced the new start/end/encoding order against Node v26.3.0
lib/buffer.js#L904-L934(comparison polarity, NaN/negative/Infinity, BigInt at 0, at length and past length, Symbol) — each branch lands where Node does. - Every
toPrimitive/toNumber/toLength/parseEncodingcall has aRETURN_IF_EXCEPTIONunder the throw scope; no raw pointers are held across them, andjsBufferToStringre-reads the livebyteLengthso a resize/detach during coercion clamps rather than overreads. - The differential fixture is deterministic (result or error class+code only), drains stdout with
Promise.all, and skips whennodeis absent.
Extended reasoning...
Overview
The PR rewrites the argument-handling prologue of jsBufferPrototypeFunction_toStringBody in /home/claude/bun/src/jsc/bindings/JSBuffer.cpp (about 40 lines) so that Buffer.prototype.toString(encoding, start, end) follows Node's order: coerce start, coerce end, return "" for an empty range, and only then parse the encoding. It removes the end == 0 early return for zero-length buffers, changes the start comparisons from >= 0 / > byteLength to > 0 / >= byteLength, and adds BigInt handling that mirrors what Node's comparison operators accept before MathTrunc throws. The tests add a describe block to /home/claude/bun/test/js/node/buffer.test.js and a differential fixture /home/claude/bun/test/js/node/buffer-tostring-args-fixture.js that is run under both Node and Bun. The two commits since my last review only shorten comments and tweak the fixture/test wording; the logic is unchanged.
Security risks
None specific to this change. The function only reads the view; jsBufferToString (JSBuffer.cpp:2126-2152) clamps offset and length against the live byteLength() after all user-observable coercions have run, and a detached buffer reports byteLength() == 0 and returns "". All user-JS entry points (toPrimitive, toNumber, toLength, parseEncoding) are followed by RETURN_IF_EXCEPTION, and no raw pointers or spans are held across them. The JSBigInt::compare calls are only reached after an isBigInt() check, and the static_cast<size_t>(fstart) only happens after fstart is proven finite, positive and below byteLength.
Level of scrutiny
Moderate. The diff is small and well-scoped, but it changes observable behavior of one of the most-used Buffer methods, deliberately diverges from Node in coercion count (documented in a one-line comment and in the tests), and removes a long-standing fast path. I traced each branch against Node's source and the results agree, and the tests pin the boundary cases (start == length, end == length for BigInt, negative/NaN/null end, empty buffer with throwing start/end). That is enough for me not to flag anything, but a maintainer should still confirm they are comfortable with the single-coercion divergence and the loss of the zero-length early return rather than treating this as a mechanical change.
Other factors
The previously noted pre-existing gaps (encoding objects with only valueOf, non-8-bit-element receivers, per-call valueOf counts, resizable-buffer shrink-vs-throw) are unchanged by this PR and were already discussed on earlier revisions; the author replied and resolved those threads. Existing coverage in buffer.test.js and the Node parallel test-buffer-tostring* tests exercise the same function, and the new differential test is hermetic (no network, no sleeps, skipIf(!nodeExe())). No CODEOWNERS entry covers the changed files. The bug hunt exited on a dry streak with no findings.
Problem
Buffer.from("abc").toString("bogus", 1, 1)throwsTypeError [ERR_UNKNOWN_ENCODING]: Unknown encoding: bogus. Node returns""for every empty range (start >= lengthorend <= start).jsBufferPrototypeFunction_toStringBody(JSBuffer.cpp:2306). It parses the encoding before it readsstartandend. Node'stoStringhandlesstart, thenend, returns''for an empty range, then resolves the encoding.startorend. Node compares them first, and a comparison accepts a BigInt.toString("utf8", 0n)works in Node.Fix
end <= startreturn. Return""forstart >= length(was>) beforeendis read.""for an empty buffer. node:buffer: fix six Node compatibility divergences #32784 did the same forindexOf.<= 0is 0, a start>= lengthreturns"", an end> lengthis the length. Every other BigInt throws aTypeError. Without this, the new order changes the error fortoString("bogus", 0n).describeintest/js/node/buffer.test.js(6 of 8 tests fail on canary 1.4.3, all pass here). One compares 1704 argument shapes under Node and Bun.Background
buf.toString(encoding, start, end)decodes the bytes in[start, end).parseEncodingthrowsERR_UNKNOWN_ENCODINGfor an unknown name. It callstoString()on an object, and thestartandendsteps callvalueOf(). User code can see the order.jsBufferToStringclamps the range to the livebyteLength, so a coercion that shrinks or detaches the buffer stays safe.Notes
Origin. No user reported this. A probe found it during the work on #43368, which lists it under "Not changed here".
Probe, Node v26.3.0 against canary 1.4.3 (b52d513).
bufisBuffer.from("abc"),emptyisBuffer.alloc(0).throws("x")is an object whoseSymbol.toPrimitivethrowsError("x").buf.toString("bogus", 1, 1),(…, 2, 1),(…, 0, 0),(…, 0, -1),(…, 0, NaN),(…, 0, null)""ERR_UNKNOWN_ENCODING""buf.toString("bogus", 3),(…, 3.5),(…, 5),(…, Infinity)""ERR_UNKNOWN_ENCODING""buf.toString(enc, 1, 1)forencin"",null,1,{}""ERR_UNKNOWN_ENCODING""buf.toString(Symbol(), 1, 1)""TypeError""buf.toString(throws("encoding"), throws("start"))startencodingstartbuf.toString("utf8", 3, throws("end"))""end""empty.toString("utf8", throws("start"))start""startempty.toString("utf8", 0, throws("end"))end""endempty.toString("utf8", 1, throws("end"))""""""empty.toString("bogus"),empty.toString(throws("encoding"))""""""buf.toString("bogus"),("bogus", 0, 1),("bogus", 1),("bogus", -1),("bogus", NaN),("", 0, 1),(null)ERR_UNKNOWN_ENCODINGbuf.toString(undefined, 1, 2)"b""b""b"BigInt rows.
TypeErrorhere has nocode.buf.toString("utf8", 0n),(…, -1n)"abc"TypeError"abc"buf.toString("utf8", 3n),(…, 2n ** 70n)""TypeError""buf.toString("utf8", 0, 4n)"abc"TypeError"abc"buf.toString("utf8", 1n),("utf8", 0, 3n),("utf8", 0, 0n)TypeErrorTypeErrorTypeErrorbuf.toString("bogus", 0n),("bogus", 1, 4n)ERR_UNKNOWN_ENCODINGERR_UNKNOWN_ENCODINGERR_UNKNOWN_ENCODINGbuf.toString("bogus", 1n)TypeErrorERR_UNKNOWN_ENCODINGTypeErrorempty.toString("utf8", 0n),(…, 1n)""""""empty.toString("utf8", 0, 0n)TypeError""TypeErrorThe fifth and the seventh BigInt rows are why the BigInt change is in this PR. The new order alone makes them a
TypeError, because Bun'stoNumber()throws for every BigInt. A first version of this change had that regression. A review of the diff found it.toLocaleStringis the same function and follows. AUint8Arrayreceiver (Buffer.prototype.toString.call(u8, "bogus", 1, 1)) follows too.The differential test.
buffer-tostring-args-fixture.jsprints one line for each of 2 buffers x 5 encodings x 14 starts x 12 ends, plus a few other shapes and the order of the first coercion of each argument. A line holds the result, or the error class andcode. The output is identical under Node v18.20.8, v20.20.2, v22.23.2, v24.21.0 and v26.3.0, so the Node version on a CI machine does not matter. Canary 1.4.3 differs from Node in 651 of the 1704 lines. This PR differs in none. The test follows thefill()test from #33033 and skips whennodeis not installed. The other seven tests need nonode.Two tests of the
describepass before and after. "throws ERR_UNKNOWN_ENCODING for a range that is not empty" pins that only an empty range skips the encoding. "coerces start and end with the number hint" pins the hint of the newtoPrimitivecall (aDatestart is a number only under that hint).A second differential script (1400 lines:
Date, boxed values, arrays, numeric strings, BigInts at and around the length, a subarray, a 70000-byte buffer, and the hint that each engine passes) also gives identical output under Node v26.3.0 and this PR. Canary differs in 502 of its lines.Cost. The zero-length early return dates from the first version of the file and has no benchmark. After this PR,
empty.toString("hex")does onetoNumber(undefined)and two compares before it returns"".empty.toString()is unchanged. A call with arguments adds two inline tag checks per argument (toPrimitiveon a value that is not a cell, andisBigInt()). I did not run a benchmark.Differences from Node that stay.
startup to three times andendtwice (start <= 0,start >= lengthandMathTrunc(start)each callvalueOf()). Bun coerces each argument once. This is deliberate and not new:maindoes the same, and so does the rest ofJSBuffer.cpp(buf.indexOf("c", obj)callsvalueOf()1 time in Bun and 3 times in Node). Node uses the value of the last coercion without a new range check, so a literal port must also throwERR_OUT_OF_RANGEfrom the native slice (anendthat returns 3 and then 100 does that in Node). A C++ comment states the difference, the in-process tests assert one coercion per argument, and the fixture compares the order of the first coercion.ArrayBufferduring a coercion, Node throwsERR_OUT_OF_RANGEfrom its native slice. Bun clamps injsBufferToString(an existing test pins that). The results of a probe withresize()andtransfer()inside each coercion are the same before and after this PR, and the debug build (ASAN) reports nothing.encoding += ''(valueOf()first),parseEncodingcallstoString()first.buf.writeandbuf.indexOfshare that helper, so this PR does not change it. Tracked in node:buffer: an encoding that is not a string is coerced differently than in Node #43426. The tests here useSymbol.toPrimitive, which both orders call.Also run with the debug build:
test/js/node/buffer.test.js(687 pass, 1 skip), and fromtest/js/node/test/parallel/:test-buffer-tostring.js,test-buffer-tostring-range.js,test-buffer-tostring-rangeerror.js,test-buffer-alloc.js,test-buffer-slice.js,test-buffer-inspect.js,test-buffer-isencoding.js,test-buffer-tojson.js,test-buffer-arraybuffer.js,test-buffer-bigint64.js,test-buffer-concat.js,test-buffer-ascii.js,test-buffer-constants.js,test-buffer-resizable.js. The new tests and the fixture also pass withBUN_JSC_validateExceptionChecks=1.Conflicts. #43368 and #30138 edit
jsBufferPrototypeFunction_writeBodyin the same file. #41498 edits theindexOfhelper. None touchestoStringBody.Self-reviewed: 7 concerns raised, 7 addressed (the BigInt regression, error messages in the tests that also matched
Unknown encoding, a Node against Bun differential test, the permalink range, inaccurate wording in this description, an imprecise BigInt comment, and a test that pins the coercion hint).The fixture prints with
console.log. In a debug build that costs about one second less thanprocess.stdout.write, and the differential test takes about 1.4 s there.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/buffer.test.js