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…g file
file-io.mdx: document the createPath option, that Bun.write overwrites a
path destination in place and truncates it, that an fd destination is
written at its position and not truncated, that the write is not atomic
(with the temp file + rename pattern), and that file.writer() opens and
creates the file at call time, writes from the start, and has no append
mode (with the fs.openSync(path, 'a') pattern). Add the options
parameter and BunFile.write() to the Reference block.
bun.d.ts: the same facts on Bun.write and BunFile.writer(). Type
BunFile.write()'s options as { createPath } (what Blob.rs do_write
reads) instead of { highWaterMark } (never read).
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Updated 3:32 AM PT - Oct 21st, 2026
❌ @robobun, your commit eb88455 has 2 failures in
🧪 To try this PR locally: bunx bun-pr 41745That installs a local version of the PR into your bun-41745 --bun |
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Status: ready for review. Docs, types and one spec. No runtime change. How each statement was checked (bun 1.4.4-canary.1+71bc8185c, the head of // file descriptor destination: written at its position, not truncated
const fd = fs.openSync(p, "r+"); // p holds "OLDCONTENT"
fs.readSync(fd, Buffer.alloc(3), 0, 3, null); // position 3
await Bun.write(Bun.file(fd), "N"); // -> 1, file "OLDNONTENT"
await Bun.write(fd, new Response("N")); // the same with a number, and with a Response, Request, BunFile or stream (16 of 16)
// path destination: replaced in place and truncated
await Bun.write(Bun.file(p), "N"); // file "N", same inode, hard link and permissions (16 of 16)
// two writes to one path that overlap are not ordered (100 rounds each)
const w1 = Bun.write(p, first), w2 = Bun.write(p, second);
await Promise.all([w1, w2]);
// first 262143 bytes, second 200000: the file holds the second text 100 times
// first 262144 bytes, second 200000: the file holds the first text 15 times
// Blob sources of 1000 and 500 bytes: the first text 4 times
// the temp file + rename example twice at once: never a mix, the first text 99 times
// each write started after the previous one settles: always the last text
// mode: 0o600 with umask 022, for a new file / an existing 0644 file
await Bun.write(p, "secret", { mode: 0o600 }); // 644 / 644
await Bun.write(p, Bun.file(src), { mode: 0o600 }); // 600 / 600
await Bun.write(p, readableStream, { mode: 0o600 }); // 600 / 644
// BunFile.write() reads createPath and type
await Bun.file(missingDir + "/x").write("x", { createPath: false }); // rejects ENOENT. TS2353 on main
await file.write("{}", { type: "application/json" }); // file.type "application/json;charset=utf-8". TS2353 on main
Bun.write(fd, "0123456789"); // runs (10 bytes). TS2769 on main
// writer(): opens and creates at call time, writes from the start, no append mode
Bun.file(missing).writer(); fs.existsSync(missing) // true, before any write
Bun.file(p).writer({ append: true }).write("N") on "OLDCONTENT" // "NLDCONTENT"
Bun.file(fs.openSync(p, "a")).writer().write("N") on "OLDCONTENT" // "OLDCONTENTN", fd still open after end()With this branch |
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughThe change documents ChangesFile writing API
Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly summarizes the main documentation and type changes for Bun.write and FileSink, including existing-file behavior and numeric file-descriptor support. |
| Linked Issues check | |
Check skipped because no linked issues were found for this pull request. |
| Out of Scope Changes check | |
Check skipped because no linked issues were found for this pull request. |
| Description check | |
The description provides the PR purpose through the Problem and Fix sections and explains verification through the reported commands, results, and detailed test notes. It does not use the exact templa… |
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Actionable comments posted: 3
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Inline comments:
In `@docs/runtime/file-io.mdx`:
- Line 173: Update the temporary-path construction around the tmp variable so
each write operation gets a unique path in the destination directory, rather
than relying only on process.pid. Preserve the existing rename flow while
preventing concurrent writes in the same process from sharing the temporary
file.
- Around line 335-336: Update the BunFile.write reference signature to match the
public declaration in bun.d.ts: add Request, BunFile, and ReadableStream to the
input union, and replace TypedArray with ArrayBufferView while preserving the
existing types and options.
- Around line 197-198: Wrap the asynchronous writer.end() call and closeSync(fd)
in a finally block so the caller-owned descriptor is closed even when
writer.end() rejects. Preserve the existing descriptor ownership behavior and
ensure closeSync(fd) executes exactly once.
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LGTM — both earlier points (unique-per-call temp filename, empty-input-truncates-fd caveat) are now addressed in the mdx and mirrored in bun.d.ts.
What was reviewed: confirmed BunFile.write in src/runtime/webcore/Blob.rs reads createPath and never reads highWaterMark, so the option-type swap matches the runtime; verified globals.ts:311:5 now points at the headers literal after the four inserted fixture lines and that no other globals.ts: line references exist in the test; checked the new prose stays in present tense / second person per docs voice.
Extended reasoning...
Overview
Docs-and-types-only PR touching four files: docs/runtime/file-io.mdx (new prose on Bun.write overwrite semantics, createPath, atomicity, and FileSink open/append behavior), packages/bun-types/bun.d.ts (JSDoc mirroring the same facts, plus correcting BunFile.write() options from { highWaterMark? } to { createPath? }), the bun-types fixture globals.ts (four new type assertions including a @ ts-expect-error proving highWaterMark is now rejected), and bun-types.test.ts (one expected-error line number bumped 307→311 to account for the inserted fixture lines). No native or JS runtime code changes.
Security risks
None. This is documentation and type-declaration surface only. The atomic-replace example was previously flagged for using process.pid (collides across concurrent in-process writers); the latest commit switched it to crypto.randomUUID(), which is unique per call and safe to copy verbatim.
Level of scrutiny
Low-to-moderate. The one substantive change — swapping BunFile.write options from highWaterMark to createPath — is a type-level correction that could break callers passing the old key, so I verified against src/runtime/webcore/Blob.rs: the do_write path reads only createPath (via get_truthy(global_this, "createPath")) and never touches highWaterMark, so the old type was simply wrong and the new one matches the runtime, per the repo's ".d.ts must mirror the runtime exactly" rule.
Other factors
Both concerns I raised on earlier pushes are addressed in commit 6ac3de7: the temp-file recipe now uses crypto.randomUUID(), and the fd-destination paragraph (mdx line 166) plus bun.d.ts:2129 now carry the empty-input-truncates caveat. I confirmed the sole hardcoded globals.ts: line reference in bun-types.test.ts was updated and that line 311 col 5 is indeed the headers key the expected TS2353 error targets. Docs prose uses present tense and second person throughout; no "will"/"simply"/"we". CodeRabbit's remaining self-resolved thread is bot-authored and two of its three threads were resolved by a non-author with a follow-up commit, so no outstanding human objections block approval.
…-io-write-semantics-docs
…tate the fd rule on each overload The two Bun.write overloads with a BunFile destination said that the destination is truncated when the input is smaller. That is false for Bun.file(fd): Bun writes at the position of the descriptor and keeps the rest of the file. Both overloads now say so. Bun.write takes a number as the destination, a file descriptor. The Reference block of file-io.mdx lists it, and the types rejected it (TS2769). The types and the destination list of the page now have it. An empty file or stream does not truncate a file descriptor destination, so the sentences about the empty input now exclude them. The page no longer gives "a" as the example flag for the descriptor: a Blob or a Response over 1024 bytes pads an O_APPEND file with NUL bytes, and a file source fails with EBADF. write.spec.ts had a FIXME test that expected a number destination to throw. It now asserts what the runtime does.
robobun
changed the title
…-io-write-semantics-docs
…applies, type `type` on BunFile.write Two Bun.write calls to one path that overlap are not ordered. A string or a TypedArray under 256 KiB is written before the call returns. Every other write completes later, so an earlier large write can finish after a later small one and the file keeps the older data. file-io.mdx and the Bun.write JSDoc now say so, and say to start a write to a path after the previous one settles. The temp file + rename example keeps the file whole and does not order two replacements. An overwritten file keeps its permissions unless `mode` sets them. Bun applies `mode` when the input is a BunFile, uses it only to create the file when the input is a ReadableStream, and ignores it for a string, a TypedArray, an ArrayBuffer, an Archive or a Blob that holds data. The `mode` JSDoc of the first overload said "If writing to a PathLike, set the permissions of the file." BunFile.write() reads `type` next to `createPath`. The options type now declares it. The page sends appends to a FileSink on a descriptor opened in append mode.
| /** | ||
| * If `true`, create the parent directory if it doesn't exist. | ||
| * | ||
| * If `false`, the write rejects when the directory doesn't exist. | ||
| * | ||
| * @default true | ||
| */ | ||
| createPath?: boolean; |
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🟡 nit (optional), pre-existing: a caller who passes the documented default createPath: true to Bun.file(fd).write() or Bun.write(Bun.file(fd), ...) gets a thrown TypeError, which this new JSDoc does not mention. write_file_internal at Blob.rs:4663-4674 throws "Cannot create a directory for a file descriptor" whenever createPath is explicitly true and the destination blob is fd-backed, while a bare number destination (now typed) silently ignores it. Fix: state on the createPath docs (bun.d.ts:2798-2805, 2176-2183, 2206-2213, 2280-2287) that the option applies to path destinations only and that true throws for Bun.file(fd), or make the runtime treat true as a no-op for every fd destination.
Why this was flagged
A caller writes await Bun.file(fd).write(data, { createPath: true }) because bun.d.ts:2798-2805 documents createPath with @ default true. do_write at Blob.rs:1283-1294 stores Some(true) in mkdirp_if_not_exists and passes a PathOrBlob::Blob whose store pathlike is PathOrFileDescriptor::Fd. write_file_internal at Blob.rs:4663-4674 matches exactly that shape and returns throw_invalid_arguments("Cannot create a directory for a file descriptor"), a synchronous TypeError, before any byte is written. Bun.write(Bun.file(fd), data, { createPath: true }) takes the same path via Blob.rs:5090-5098. Bun.write(fd, data, { createPath: true }) with a plain number arrives as PathOrBlob::Path(Fd) and is not caught by that check, so it writes normally. The runtime did this on the base branch too, but the base types did not accept a number destination and the BunFile.write() options type had no createPath, so the contradiction between the documented default and the throw is new to readers of this PR's JSDoc.
Verification: nit. Trigger: a TypeScript caller passes the documented default explicitly, Bun.file(fd).write(data, { createPath: true }), where the BunFile is fd-backed. write_file_internal at Blob.rs:4663-4673 returns throw_invalid_arguments("Cannot create a directory for a file descriptor") before any write. The new JSDoc this PR adds at packages/bun-types/bun.d.ts:2798-2805 says nothing about this.
| it("Writes to the file descriptor when `destination` is a number", async () => { | ||
| using dir = tempDir("bun-write-fd", { "file.txt": "0123456789" }); | ||
| const filepath = path.join(String(dir), "file.txt"); | ||
| const handle = await fs.open(filepath, "r+"); | ||
| try { | ||
| expect(await Bun.write(handle.fd, "abc")).toBe(3); | ||
| } finally { | ||
| await handle.close(); | ||
| } | ||
| expect(await fs.readFile(filepath, "utf-8")).toBe("abc3456789"); | ||
| }); |
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🟡 nit (optional): the only runtime test for a number destination writes at position 0 with a string, so it cannot fail if Bun ignored the descriptor's position, and it exercises none of the other overloads this PR adds number to. The types now accept Bun.write(fd, Response) and Bun.write(fd, BunFile) (bun.d.ts:2203, 2263) and the docs promise a write "at the descriptor's current position" (file-io.mdx:167), but no test seeks before writing or uses those inputs. Fix: add cases alongside this one that open with "r+", move the position (for example handle.write of a prefix or fs.readSync with a position), then write a string, a Response and a Bun.file() through the number and assert exact file bytes.
Why this was flagged
test/js/bun/bun-object/write.spec.ts:22-32 opens "file.txt" holding "0123456789" with "r+" (position 0), writes "abc" and asserts "abc3456789". A runtime that always wrote at offset 0 and never honoured the descriptor position would pass this test, so the "writes at its position" comment at line 21 is not what the assertion proves; only no-truncation is proved. The PR changes bun.d.ts:2203 and bun.d.ts:2263 so TypeScript now accepts a number for Response/Request and BunFile inputs, which go through different runtime paths (WriteFile / WriteFileWaitFromLockedValueTask at Blob.rs:4946-4972, CopyFile at Blob.rs:4417-4425) than the string fast path at Blob.rs:4712-4717 that this test hits, and none of those paths has an automated test with a number destination. The base branch had an it.failing placeholder here; the PR replaces it with one case where REVIEW.md asks for every overload and the stated invariant.
Verification: nit. A future regression in numeric-fd handling (writing at offset 0 instead of the descriptor's position) would pass the suite. test/js/bun/bun-object/write.spec.ts:21-32 seeds "0123456789", opens with fs.open(filepath, "r+"), calls Bun.write(handle.fd, "abc") and asserts "abc3456789". An implementation that pwrite()s at offset 0 regardless of position produces the identical result.
| /** | ||
| * Incremental writer for files and pipes. | ||
| * | ||
| * For a path-backed `BunFile`, this opens the file immediately and creates | ||
| * it if it does not exist (the parent directory must exist). Writes start | ||
| * at the beginning of the file. There is no append mode: to append, open | ||
| * the file with `fs.openSync(path, "a")` and call `Bun.file(fd).writer()`. | ||
| * The writer does not close a file descriptor that you passed in. | ||
| * | ||
| * @example | ||
| * ```ts | ||
| * const writer = Bun.file("output.txt").writer(); | ||
| * writer.write("hello\n"); | ||
| * await writer.end(); | ||
| * ``` | ||
| */ | ||
| writer(options?: { highWaterMark?: number }): FileSink; |
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🟣 pre-existing, not blocking: nit, pre-existing: TypeScript users still get a highWaterMark option on BunFile.writer() that the runtime never reads, while this PR removes the same unread key from BunFile.write(). The rewritten JSDoc at bun.d.ts:2737-2753 keeps writer(options?: { highWaterMark?: number }), but Blob.rs:1741-1754 parses the options with the FileSink tag, which reads only path and fd (streams.rs:205-248), and FileSink.rs has no high-water or chunk-size field. Fix: make the declaration and docs match the runtime: either drop highWaterMark from writer() (and the file.writer({ highWaterMark: 1024 * 1024 }); // 1MB example at docs/runtime/file-io.mdx:226 and the reference block at file-io.mdx:343), or state in the JSDoc that it currently has no effect.
A small fix can ride a push you are already making; otherwise a short reply is enough.
Why this was flagged
A user calls Bun.file("out.txt").writer({ highWaterMark: 1024 * 1024 }) as docs/runtime/file-io.mdx:226 shows. Blob.rs:1741-1745 turns the options object into streams::Start::from_js_with_tag::<{ StartTag::FileSink }>, whose FileSink arm (streams.rs:205-248) only looks at path and fd and otherwise returns default FileSinkOptions. file_sink::Options (FileSink.rs:163-170) has input_path, mode, truncate, mkdirp and nothing else, and FileSink.rs contains no highWaterMark, high_water_mark or chunk_size reference. So the buffer size the user configured changes nothing. This is the same situation the PR fixes for BunFile.write() (it deletes { highWaterMark?: number } there because do_write never reads it), but the sibling writer() declaration the PR rewrites the JSDoc for keeps the unread option. The base branch has the same declaration, so this is pre-existing; the PR's stated rule that declarations mirror the runtime is what makes it in scope.
Verification: pre-existing (nit). Trigger: any TypeScript caller writing Bun.file(p).writer({ highWaterMark: n }). packages/bun-types/bun.d.ts:2753 still declares writer(options?: { highWaterMark?: number }): FileSink;. The FileSink arm at src/runtime/webcore/streams.rs:205-248 reads only path and fd. The base branch has the identical writer signature and docs, so merging makes nothing worse.
Problem
Bun.writedoes to an existing file, or that overlapping writes to one path are not ordered. Bun writes a string under 262,144 bytes inside the call (Blob.rs:4692) and other data later. Writing 262,144 bytes, then 200,000, leaves the first text in 15 of 100 rounds.Bun.write(fd, data)(TS2769), which works.BunFile.write()options are typed{ highWaterMark }. The runtime readscreatePathandtype(Blob.rs:1262).modeJSDoc says "set the permissions of the file". With a string,{ mode: 0o600 }gives 0644.Fix
file-io.mdx: a section "Overwriting an existing file",createPath, and what.writer()does when called.bun.d.ts: the same facts. The destination takes anumber.BunFile.write()takes{ createPath?, type? }. ThemodeJSDoc says whenmodeapplies.write.spec.ts: the FIXME test that expected a number to throw asserts the write.test/integration/bun-types/bun-types.test.ts,test/js/bun/bun-object/write.spec.ts.Background
Bun.writeis called on a typedarray-backed Blob #18600 tried to reject a number destination. The review said: "People use file descriptor numbers. It should be allowed."fsPromises.writeFile().Downsides
BunFile.write(data, { highWaterMark })is now a type error. The key never had an effect.Bun.write(n, data)compiles, so tsc no longer catches a number passed by mistake.modefor a string. Bun.write: applymodefor string, buffer, Blob and stream sources on POSIX #32885 changes that. Checked on Linux only.Notes
Runs marked "main" are on 1.4.4-canary.1+71bc8185c. The other runs are on 1.4.4-canary.1+6bb5dc195 and 1.4.3-canary.1+367d939d9 (same results). Linux x64, types with
tsc7.0.2.Two writes to one path that overlap (main, 100 rounds each, no
awaitbetween the two calls; the file holds the second text / the first text / a mix):1.4.3-canary.1+367d939d9 gives 100 / 0 / 0, 86 / 14 / 0, 98 / 2 / 0 and 100 / 0 / 0 for the four string rows. The shares move with the load on the machine. The step at 262,144 bytes does not.
Bun.writereturns (main): a string orUint8Arrayof 262,143 bytes is on disk in full. At 262,144 bytes the file does not exist yet.Bun.write(file, JSON.stringify(state))(main, 200 waves of 4 requests): with a 261 KB file, 200 waves end with the right file. With a 263 KB file, 65 are right, 126 are stale and 9 are not JSON (second run: 83, 114, 3). With each write started after the previous one settles, 200 are right at 263 KB and at 1 MB.fsdocs: "It is unsafe to usefsPromises.writeFile()multiple times on the same file without waiting for the promise to be settled."Bun.write. That is a runtime design change and is not in this PR.numberfrom the Reference block instead. That keeps a supported call a type error.mode(main and 1.4.3 agree; umask 022,{ mode: 0o600 }; a new file / an existing 0644 file):Without
mode, an overwrite keeps the inode, the hard link and the permissions (main, 16 of 16 cases: 8 input kinds, path andBun.file(path)destinations, a 0640 file with a second link).typeonBunFile.write()(main):file.write("{}", { type: "application/json" })setsfile.typetoapplication/json;charset=utf-8, andnew Response(file)sends thatContent-Type. A secondBun.file(path)keeps its own type. A string that is not a MIME type is ignored. A number throwsERR_INVALID_ARG_TYPE.Bun.write(file, data, { type })does not read the key.File descriptor destination (
fs.openSync(path, "r+"), file holds "OLDCONTENT", position moved to 3, write "N"):The same holds over a 200,000 byte file with inputs of 10, 1,025, 5,000 and 70,000 bytes (112 cases).
Bun.file(path)and a path string leave "N".Empty input to a file descriptor
This is why the sentences about the empty input say "that is not a file or a stream" (and "an empty body that is not a file" on the
Responseoverload). #32890 (open) removes the truncation.Types
With the number in the types and the old
write.spec.ts,cd test && bun checkreports four new errors (TS2578, unused@ts-expect-error, lines 23, 25, 27 and 34). That is why the spec changed. With the new spec the error list is the same as onmain.Other checks
BunFile.write():{ createPath: false }with a missing parent rejects withENOENT. The default creates the parent.Bun.write(dest, Uint8Array)of 20 MiB + i * 99,991 bytes. 4 of 10 rounds end with two payloads mixed, for example 21,371,484 bytes of one and 99,991 of the next..writer(): creates the file at call time, a missing parent isENOENT, "N" over "OLDCONTENT" gives "NLDCONTENT",{ append: true }and{ flags: "a" }change nothing.Bun.file(fs.openSync(path, "a")).writer()appends correctly and leaves the descriptor open afterend()(main, 27 of 27 cases: 14 bytes to 5 MiB, as a string, as bytes and in 4 KiB chunks). The page sends appends there. WithBun.writeon anO_APPENDdescriptor (main), aBlobover 1024 bytes, aResponse, or a string of 256 KiB or more pads the file with NUL bytes (Bun.write(Bun.stdout, data) pads an appended file with NUL bytes when data is over 1024 bytes (fallocate on an O_APPEND fd) #44387, fix in Keep the file size when preallocating a write #44425), and aBun.filesource rejects withEBADFfromcopy_file_range(fix in Bun.write: copy a file to an O_APPEND destination fd with the read/write loop #41685). A smaller string orTypedArrayappends correctly.write.spec.ts: theit.failingtest came from fix: Bun.write() with empty string creates a file #18561. The "if/when we ban fds" note came from fix: assertion failure whenBun.writeis called on a typedarray-backed Blob #18600, the PR whose review kept number destinations.modefor string, buffer, Blob and stream sources on POSIX #32885 addsmodeto the options ofBunFile.write(), in the same object that this PR edits. The PR that lands second resolves that object to{ createPath?, type?, mode? }with themodeJSDoc of Bun.write: applymodefor string, buffer, Blob and stream sources on POSIX #32885, and updates the sentences aboutmodeinfile-io.mdxand on the firstBun.writeoverload.modefor string, buffer, Blob and stream sources on POSIX #32885 (mode), Bun.write: stop truncating a caller's fd (stdout, stderr, Bun.file(fd)) on an empty source #32890 (empty source to a descriptor), Honor Bun.file().slice() bounds in Bun.write #41209 (slicedBun.filesources), Bun.write: copy a file to an O_APPEND destination fd with the read/write loop #41685 and Keep the file size when preallocating a write #44425 (O_APPENDdescriptors).Bun.writeis the open request Support appending files withBun.write#10473. An offset option forFileSinkis Add offset option to FileSink #5821.