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…le and fs.promises.readFile
Node reads the flag of these calls as `options.flag || default`, so
`{ flag: append && "a" }` with `append = false` writes with the default
flag. Bun passed false, 0, NaN and 0n to the strict flag parser.
- FileSystemFlags::from_js_or_default is the one place for that rule.
- The writeFile and appendFile parser uses it for the sync, callback and
promise forms.
- fs.promises.readFile uses it through the promise arm of the async
binding. fs.readFileSync and fs.readFile(cb) stay strict, as in Node.
- fs.promises.writeFile with iterable data uses `||` for the flag.
…pper The native read parser and the async binding are as on main again. - fs.promises.readFile reads `options.flag` once. When a flag is given it passes a copy of encoding, signal and `flag || 'r'` to the native call. - fs.promises.writeFile with iterable data ignores an argument after the options, as Node does. - Tests: a valid flag still reaches the open call, and a truthy flag getter is read once.
The wrapper reads the flag from its second parameter. It no longer builds a rest array and no longer spreads it into the native call.
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Updated 9:05 AM PT - Oct 20th, 2026
❌ @robobun, your commit 53f2d96 has some failures in 🧪 To try this PR locally: bunx bun-pr 45645That installs a local version of the PR into your bun-45645 --bun |
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WalkthroughFilesystem write and append APIs now use default flags when supplied flags are falsy. The changes include async-iterator ChangesFilesystem flag defaults
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to
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| Title check | |
The title clearly and concisely describes the main change: treating falsy flags as the default in Node.js filesystem write and append APIs. |
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The description explains the problem, implementation, scope, known differences, and verification results. It provides the required change summary and test information, although it uses Problem and Fix… |
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The description explains the problem, implementation, scope, known differences, and verification results. It provides the required change summary and test information, although it uses Problem and Fix headings instead of the template headings.
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Inline comments:
Review comments at @src/js/node/fs.promises.ts:
- Around line 314-316: Update the options handling before `_readFile` to always
create a new options object from the captured `flag`, even when it is undefined,
so `args::ReadFile::from_js` cannot reread the original getter; preserve the
existing encoding and signal values and apply the default read flag to the
captured value.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked whether the rebuilt { encoding, signal, flag } object in the fs.promises.readFile wrapper loses anything the native call needs — ReadFile::from_js in src/runtime/node/node_fs.rs reads only those three properties, so the copy is lossless for the native parser. The flag: 0 truncation change in writeFile/appendFile matches Node's options.flag || 'w' and is called out in the PR.
Extended reasoning...
The change touches the shared WriteFile/AppendFile argument parser in src/runtime/node/node_fs.rs, a new FileSystemFlags helper in src/runtime/node/types.rs, and two JS wrappers in src/js/node/fs.promises.ts, plus a large new test block; it touches no security-sensitive surface. Inline findings were posted, so a human look is warranted regardless; the note above records one additional concern examined and ruled out.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/js/node/fs.promises.ts— pre-existing: callers of fs.promises.writeFile with iterable data still get a stray 4th or 5th argument used as the open flag or mode, which Node ignores. The diff stops this only for the object-options flag at src/js/node/fs.promises.ts:1625; the string/undefined branch keepsflag ??= "w"andmode ??= 0o666at src/js/node/fs.promises.ts:1633-1634, and the object branch keepsmode ?? (mode || 0o666)at src/js/node/fs.promises.ts:1626. Fix: derive flag and mode only from optionsOrEncoding on every branch and drop the positional flag and mode parameters of writeFileAsyncIterator, which covers the 3 sites listed. Same pattern at 3 sites (src/js/node/fs.promises.ts:1626, src/js/node/fs.promises.ts:1633, src/js/node/fs.promises.ts:1634). [also at: src/js/node/fs.promises.ts:1633 - nit, pre-existing: callers of iterable fs.promises.writeFile that pass a string encoding and a fourth argument still get that argument used as the open flag, which Node ignores.]Why this was flagged
A user calls fs.promises.writeFile(path, ["new"], "utf8", "a"). exports.writeFile at src/js/node/fs.promises.ts:331 forwards every rest argument to writeFileAsyncIterator(fdOrPath, iterable, optionsOrEncoding, flag, mode) at src/js/node/fs.promises.ts:1619. Because optionsOrEncoding is a string or undefined, the branch at src/js/node/fs.promises.ts:1631 runs and
flag ??= "w"at :1633 keeps the caller's "a", so fs.open at :1645 opens for append and the existing content survives; Node's promises.writeFile(path, data, options) ignores a fourth argument and truncates with 'w'. The same holds for a fifth positional mode on both branches (:1626 and :1634). The base branch behaves identically on these three lines; the diff only rewrote the object-branch flag at :1625 and the new test in test/js/node/fs/fs-promises-writeFile-async-iterator.test.ts only exercises the object-options case, so the string/undefined branch remains untested and divergent.Verification: pre-existing, triggered when a caller passes a 4th (or 5th) positional argument to fs.promises.writeFile with iterable data and a string/nullish options argument. The sibling branch at src/js/node/fs.promises.ts:1631-1634 is untouched:
flag ??= "w"; mode ??= 0o666;, sofs.open(fdOrPath, flag, mode)at :1645 opens for append where Node yields "new". Base branch fails identically on the string branch.
…alsy-flag-default
The wrapper now copies `encoding`, `signal` and `flag` for every options object, so the native parser never reads the caller's object a second time. `FileSystemFlags::from_js_if_truthy` returns an `Option`, and the caller applies the default.
…rser The JS wrapper read `options.flag` with a plain property read, so a `flag` on `Object.prototype` reached the open call. The native parser stops at `Object.prototype`. The wrapper is back to its form on main. The promise arm of the async binding now selects `ReadFile::from_js_promises`, which parses the flag with `FileSystemFlags::from_js_if_truthy`.
The fs.promises.readFile rule needs a per-arm parser hook. That mechanism belongs in a change of its own, so the read parser and the async binding are the same as on main again. - The write parser filters a falsy flag at its one read. - The iterable path of fs.promises.writeFile keeps its positional fallback and changes only the operator. - fs-write-flag-fixture.js prints what each writeFile and appendFile form does with each flag value. fs.test.ts runs it under Node.js and under Bun and compares the output.
robobun
changed the title
flag as the default in writeFile, appendFile and fs.promises.readFileflag as the default in writeFile and appendFile
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On the review note about the positional
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On the note "confirm that the earlier undefined-getter concern no longer applies": it no longer applies.
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…alsy-flag-default
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The fd and FileHandle case from "Not changed here" is ready on the branch
It is not a stacked PR, because a stacked PR merges into this branch. I will open it against main after this PR merges. |
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2 participants
Problem
fs.writeFileSync(file, text, { flag: append && "a" })withappend = falsethrowsERR_INVALID_ARG_VALUE: The argument 'flags' is invalid. Received falseand writes nothing. Node writes with the default flag.WriteFile::from_js_with_default_flag(src/runtime/node/node_fs.rs:4261) reads the flag withget_truthy, which filters onlyundefined,nulland"".false,0,NaNand0nreach the strict parser.Fix
writeFileandappendFileforms, keeps its default flag for a falsy value.fs.promises.writeFileparses its options in JS: its flag line changes from??to||.node:fsarguments.test/js/node/fs/fs.test.ts. 21 of 24 new tests fail without the fix. One compares Node.js and Bun output.Background
options.flag || 'w'inwriteFileand!options.flaginappendFile: a falsy flag means "not given".get_truthyitself:Bun.serve({ port: 0 })needs0to pass.fs.promises.readFilehas the same rule in Node, but only in the promise form, so it is a separate change.Downsides
flag: 0(O_RDONLY) now means "not given" inwriteFileandappendFile, as in Node.writeFileSync(existing, "", { flag: 0 })now truncates..text: +254 bytes.fs.promises.readFilerejectsfalse,NaNand0n. An fd or FileHandle target rejects an invalid truthy flag.Notes
Repro. The report's script writes with
{ flag: append && "a" },append = false, throughwriteFileSync,appendFileSync,writeFileandappendFile, and reads the file back. On main each of the 4 writes throwsERR_INVALID_ARG_VALUEand the file keeps its old content. Under Node v26.3.0 and on this branch each write lands.Origin. Fuzz ledger note #503697, relayed by a maintainer. The note reports the same failure on 1.3.14 and on 1.4.0 to 1.4.3. There is no GitHub issue and no user report. The note covers
writeFileandappendFileonly. The rule is "Never validate stricter than Node" (.claude/docs/landing-prs.md). #32966 left the per-function falsy-flag handling for a follow-up. It assumed that Bun's write rule already matchedoptions.flag || 'w'. It matches only forundefined,nulland''.Both outputs.
test/js/node/fs/fs-write-flag-fixture.jsprints one line for each outcome, with the flag values that give it. Node v26.3.0 and this branch print the same 44 lines. The first group:Main prints this for the same group:
Node source (v26.3.0).
fs.writeFile: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2343fs.writeFileSync: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2398fs.appendFile: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2461fs.appendFileSync: https://github.com/nodejs/node/blob/v26.3.0/lib/fs.js#L2485fs.promises.writeFile: https://github.com/nodejs/node/blob/v26.3.0/lib/internal/fs/promises.js#L1889fs.promises.appendFile: https://github.com/nodejs/node/blob/v26.3.0/lib/internal/fs/promises.js#L1922Rows that differ from Node v26.3.0. One script, 224 rows: 6 write entry points x path / fd / FileHandle x 14 flag values (
undefined,null,'',false,0,-0,NaN,0n,true,'bogus',{},'a','w',1), the threereadFileforms x the same flags, andfs.promises.writeFilewith iterable data x the 8 falsy flags. Release builds of main ed58ba2 and of this branch.fs.promises.writeFilefs.promises.readFilereadFileSync/fs.readFile(cb)Node itself aborts on
flag: -0inreadFileSyncandfs.readFile(Assertion failed: args[1]->IsInt32()). Those 2 rows are left out.Measurements. Release builds (
bun scripts/build.ts --profile=release) of main ed58ba2 and of this branch, same tree, same build directory. Instruction counts:gdbsingle-step over one call of a JSC host function, entry to return, call 100 of a fixed loop,BUN_JSC_useJIT=0. Both binaries run from paths of the same length, because the stack position changes the counts of the SIMD string routines.WriteFile::from_js_with_default_flag(1817 to 1967 bytes). Every other function has the same size, which includes the read parser, everyNodeJSFSPrototype__*host function and everyrun_asyncmonomorph.writeFileSync/appendFileSync, this branch - main, instructions per call: no options 0,'utf8'-1,{}+2,{encoding:'utf8'}+2,{flag:'w'}+18,{flag:1}+10 (of about 3,000 to 4,200).fs.promises.writeFile: 0 and +18.readFileSync: 0 in each of 4 option shapes.writeFileSynccall (mi_mallocentries inside the host call):{}4 to 4,{flag:'w'}4 to 4,{flag:false}on this branch 4.writeFileSync(path):{flag:'w'}openat, write, ftruncate, closeon both builds.{flag:false}and{flag:0}on this branch give the same 4. Main with{flag:0}:openat, write (EBADF), close..text: +254 bytes (size, 106,749,925 to 106,750,179).fs.promises.js: 43,565 to 43,563 bytes.fs.jsidentical.options.flagreads per call: 1 on each of 7 entry points for afalse, a'r+'and anundefinedgetter, as in Node v26.3.0 (asserted infs.test.ts).Iterable data.
fs.promises.writeFile(path, iterable, options)has its own option parser in JS (writeFileAsyncIterator). Only the operator of the flag line changes:options.flag ?? (flag || "w")is nowoptions.flag || flag || "w". The positionalflagandmodearguments of that function behave as on main. #39186 removes them. #39186, #36485 and #43368 edit the same function or its test file. In a conflict with #39186 the flag line resolves to its|| defaultFlag.Not changed here.
fs.promises.readFilewithflag: false,NaNor0n(3 rows of the table): Node reads (options.flag || 'r'), Bun rejects.fs.readFileSyncandfs.readFile(cb)are strict in Node too, so the read parser must differ between the promise form and the callback form. An earlier push of this PR carried that (commit 96a3138 in this branch). It is now a separate change, because it adds a hook to the parser trait of every asyncnode:fscall.true,'bogus',{}): Node ignores the flag when it opens no file, Bun throws. 18 rows of the table. Two Node forms do check it:fs.writeFileSync(fd, string, { encoding: 'utf8' })andfs.readFileSync(fd, { encoding: 'utf8' })take a fast path that converts the flag first. Bun already matches those two.readFileSyncandfs.readFile(cb)withflag: '': Bun reads, Node throwsERR_INVALID_ARG_VALUE. 2 rows of the table.mode: ''inwriteFilehas the same cause (get_truthyfilters the empty string).fs.readFile(path, cb)andfs.writeFile(path, data, cb)pass the callback as the options, and the native parsers readencoding,flagand the other options from it. Node uses the defaults there. Withcb.encoding = 'utf8', Bun gives a string and Node gives a Buffer.flaginherited fromObject.prototypereaches it, as in Node. The natively parsed forms ignoreObject.prototype. With a Buffer or URL path the iterable path ignoresflagandmodeand does not truncate (fs.promises: accept iterable data in appendFile, like writeFile and node #39186).flag:path.format({ root: false, base: 'b.txt' })gives'falseb.txt'(Node:'b.txt', node:path: rewrite on JSString storage in Rust, match Node.js v26 exactly, ~3x faster #37305).fs.watch(dir, { recursive: 0 })throwsERR_INVALID_ARG_TYPE(Node watches).Related open PRs. #39186, #36485 and #43368 (iterable
fs.promises.writeFile). #39274 and #41928 edit the same native parsers. #39330 and #39335 change other option reads ofnode:fstoward Node.Tests. The new
describeblock has 24 tests. Without the fix 21 fail. The 3 that pass are guards: an invalid truthy flag still throws, and a'r+'and anundefinedflag getter are read once. The Node.js comparison is skipped on a machine with nonode. This PR adds 0 passing Node tests: Node's own suite passes a falsy flag once,{ flag: null }intest-fs-promises-writefile.js, and that test passes on main.Suites run with the debug build.
fs-promises-writeFile-async-iterator.test.ts,promises.test.js,dir.test.ts,cp.test.ts,fs-path-length.test.ts,fs-mkdir.test.ts,glob.test.ts: all pass. 336 vendoredtest/js/node/test/parallel/test-fs-*tests: all pass.fs.test.ts: 636 pass, 5 fail. The 5 are fourreaddir ... x 100tests, which exceed their 10 s limit under debug + ASAN in my container, and onecrypto.pbkdf2child-process test that timed out in the same run and passes alone.cargo clippy -p bun_runtimeandtsc -p src/js/tsconfig.jsonpass.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/fs/fs.test.ts