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| Layer / File(s) | Summary |
|---|---|
Prompt contract and reading src/runtime/webcore/prompt.bind.ts, src/runtime/webcore/prompt.rs |
The binding declares optional string messages and defaults, and a nullable string result. The implementation returns the selected default for empty input and returns no value on read or output failures. |
Prompt behavior validation test/js/web/web-globals.test.js, docs/runtime/secrets.mdx |
Subprocess tests cover input decoding, long answers, EOF, empty lines, CRLF, defaults, and argument conversion. The secrets example throws when the prompted token is empty. |
Suggested reviewers: jarred-sumner
Priority:
Severity of issue fixed: Low
Merge Risk: 
47f13
-
The prompt behavior is covered by the new tests, and no merge-blocking issue was found. Merge after normal checks.
Pre-merge checks: 
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Check name Status Explanation Resolution Out of Scope Changes check The PR changes confirm(undefined)to displayConfirminstead ofundefined. The change comes fromb.string.loose.utf8.ifPassedto.optionalinsrc/runtime/webcore/prompt.bind.tsand has a d…Remove the unrelated confirm(undefined)behavior change and its dedicated test. Preserve the previousconfirm()argument semantics unless an active linked issue requires the new behavior.-
Check name Status Explanation Title check The title names the two APIs and the schema change, and it states the visible result (a bare Enter returns a string). It is long, at 92 characters, but it is specific and matches the main change. The … Description check The description follows the template. It has a problem statement, a fix, and a verification section, and it covers the test runs and the build results. The template asks for 'What does this PR do?' an… Linked Issues check Issue #43983requiresprompt()to return""for a bare Enter andnullfor EOF.src/runtime/webcore/prompt.rsuses an empty string when no default is supplied, returns the default string for an…Full details: Title check
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Explanation
The title names the two APIs and the schema change, and it states the visible result (a bare Enter returns a string). It is long, at 92 characters, but it is specific and matches the main change. The PR changes the signatures in prompt.bind.ts, and the Rust body returns a string on an empty line.
Full details: Description check
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Explanation
The description follows the template. It has a problem statement, a fix, and a verification section, and it covers the test runs and the build results. The template asks for 'What does this PR do?' and 'How did you verify your code works?', and both are present. The description is long and dense, and it includes a large collapsed notes block. The verification section also states that the platform-specific tests did not run locally and defers to CI.
Full details: Linked Issues check
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Explanation
Issue
#43983requiresprompt()to return""for a bare Enter andnullfor EOF.src/runtime/webcore/prompt.rsuses an empty string when no default is supplied, returns the default string for an empty line, and returnsNonefor EOF and read failures.src/runtime/webcore/prompt.bind.tsconverts the default to a string and declares a nullable string result.test/js/web/web-globals.test.jscovers empty lines, CRLF, defaults, EOF, and conversions.
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Full details: Out of Scope Changes check
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Explanation
The PR changes
confirm(undefined)to displayConfirminstead ofundefined. The change comes fromb.string.loose.utf8.ifPassedto.optionalinsrc/runtime/webcore/prompt.bind.tsand has a dedicated test. Issue#43983coversprompt()and does not require thisconfirm()behavior change. Theprompt()tests, implementation, and the secrets example are connected to the issue.Resolution
Remove the unrelated
confirm(undefined)behavior change and its dedicated test. Preserve the previousconfirm()argument semantics unless an active linked issue requires the new behavior.
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Inline comments:
In `@src/runtime/webcore/prompt.rs`:
- Around line 239-240: In the prompt handling flow, convert the message with
arguments[0].to_utf8(global) before converting the default argument with
to_js_string(global). Move the default conversion below the message conversion,
preserving a single default conversion and the existing behavior for absent or
undefined defaults.
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Updated 7:03 PM PT - Oct 26th, 2026
✅ @robobun, your commit 47f13d9cb19052f4868778795d6f9bc24bfaf441 passed in 🧪 To try this PR locally: bunx bun-pr 43985That installs a local version of the PR into your bun-43985 --bun |
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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 holding the default's &JSString across the blocking stdin read in src/runtime/webcore/prompt.rs is GC-safe — the cell pointer is a stack local visible to the conservative scan (same mechanism JSString::ensure_still_alive relies on) and no JS runs between conversion and return, so it is not at risk. Conversion order is also correct: to_utf8 on the message runs before to_js_string on the default, and both propagate a throwing toString via ? before any output is written.
Extended reasoning...
The change rewrites the argument-conversion and empty-line branch of the Web prompt() host function in src/runtime/webcore/prompt.rs and adds a 10-row test.each table in test/js/web/web-globals.test.js; it touches no security-sensitive surface. Two verified findings are posted inline (the explicit-undefined message inconsistency and the runPrompt exitCode-first assertion), and a further verified finding was withheld, so approval is off the table; the ruled-out GC-lifetime and conversion-order concerns are noted so a human reviewer need not re-derive them.
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LGTM — the two points from the earlier pass (undefined message, test helper asserting output before exit code) are addressed in 7d98370, and the remaining diff is a small, spec-driven change with a table that covers it.
What was reviewed:
- GC rooting of the
Option<&JSString>default held across the stdout/stdin I/O in src/runtime/webcore/prompt.rs: both coercions run before any I/O so no user JS runs after it is created;view()is the only later allocation and its guard drops throughensure_still_alive, matching theJSStringViewpattern in src/jsc/JSString.rs. - Return-path diff old vs new: only the LF / CRLF empty-line branches change (
""or the coerced default instead ofNULL/ raw arg); EOF, read error, and typed-answer paths are unchanged. alert()left as-is is correct — the HTML IDL declares it as an overload, notoptional DOMString, soalert(undefined)printing "undefined" matches browsers.bstris still used by other files in the runtime crate, so nothing went dead.
Extended reasoning...
The change touches src/runtime/webcore/prompt.rs (about 40 lines: WebIDL optional-argument handling for prompt() and confirm(), coercions moved ahead of stdout writes, empty-line result switched from JSValue::NULL to the coerced default or the empty string) and adds a 15-row describe.concurrent table plus two confirm rows in test/js/web/web-globals.test.js. It touches no security-sensitive surface; the only unsafe use (the process-global stdin reader) is pre-existing. Approve was decided by the small size, the verified API surface (to_js_string, view, to_js, js_empty_string, Display for JSStringView all exist with the assumed semantics), the argument-order test that pins the coercion sequencing, the absence of CODEOWNERS coverage on the changed paths, and the fact that every prior thread has a follow-up commit; the only third-party comments are from bots. The user-visible change (bare Enter now returns "" rather than null) is spec- and browser-conformant and is documented in the PR's Downsides section along with the #36369 landing-order note.
prompt.bind.ts declares the default as `b.string.loose.optional` and the result as `b.string.nullable`. The generated code converts the message and then the default before the body runs. The body takes and returns `Option<bun_core::String>`. An empty line returns the default as a string, and the empty string when no default is given. `null` is returned only at the end of the input. Before, an empty line returned the second argument as it was, and `null` without one. The example in docs/runtime/secrets.mdx stops on an empty answer before it stores the token.
The message of `confirm()` and `prompt()` is `optional DOMString message = ""`, so the schema declares it `.optional`: `prompt(undefined)` prints `Prompt ` and `confirm(undefined)` prints `Confirm [y/N] `. Before, they printed the text `undefined`. `alert()` and `alert(message)` are overloads, so `alert` keeps `.ifPassed` and `alert(undefined)` still prints `undefined`.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The two newest commits address the points I raised earlier (the helper now asserts a combined { stdout, stderr, exitCode } object, and an undefined message is treated as absent for prompt()/confirm() while alert() keeps .ifPassed).
Beyond the inline finding, I also checked: the message byte slice stays valid while the defaultValue ToString runs, since the generated binding converts arguments in order through the WebCore IDL converters and holds them in its own frame; a Latin-1 or UTF-16 default is printed via to_utf8 in Display for bun_core::String (src/bun_core/string/mod.rs:1146), not as raw bytes; and packages/bun-types/globals.d.ts:1011 already declares prompt(message?: string, _default?: string): string | null, so no types change is needed. Since bare Enter now returns "" instead of null for existing scripts, a maintainer should still weigh that compatibility call.
Extended reasoning...
The change rewires the bindgen schema for alert/confirm/prompt in src/runtime/webcore/prompt.bind.ts, drops the JsScope/JSValue plumbing from prompt() in src/runtime/webcore/prompt.rs so it takes Option<bun_core::String> and returns Option<bun_core::String>, adds two test.concurrent.each tables with a runDialog subprocess helper in test/js/web/web-globals.test.js, and adds a null-guard line to docs/runtime/secrets.mdx. It touches no security-sensitive surface; stdin bytes are copied with bun_core::String::clone_utf8 and write failures all return None. A pre-existing inline finding is being posted, my two earlier inline comments were addressed by the latest commits, and the user-visible semantic change (empty line returns "" rather than null, default coerced to a string) is a compatibility decision a maintainer should confirm, so defer rather than approve.
Fixes #43983
Problem
prompt("Q?")returnsnullfor a bare Enter, the same value as the end of the input. A loopwhile ((line = prompt()) !== null)stops at the first empty line.src/runtime/webcore/prompt.rs:302and:309,return Ok(default.unwrap_or(JSValue::NULL)). A given default comes back unconverted:prompt("Q", 5)gives the number5.Fix
prompt.bind.tstypes the default (b.string.loose.optional) and the result (b.string.nullable). An empty line returns the default as a string, or""without one.nullis only the end of the input.undefinedis the default message ofprompt()andconfirm()(.optional).alertkeeps.ifPassed.test/js/web/web-globals.test.js, 30 new rows, main fails 14. Alsobindgen.test.ts,bun-types.test.ts,rust:check-all.Background
src/codegen/bindgen.tsgenerates the argument conversions fromprompt.bind.ts..ifPassed: absent only when the call has no such argument..optional:undefinedis absent.prompt(optional DOMString message = "", optional DOMString default = "").nullmeans that the user aborted. Browsers and Deno return""for an empty answer.unwrap_or("")at the two lines:prompt("Q", null)plus Enter still returnsnull. Considered a conversion in the body: the result stays untyped.Notes
Before and after (release builds of main 449877f and of this diff, stdin is a pipe)
prompt("Q?"), Enternull""prompt("Q?"), end of inputnullnullprompt("Q?", "d"), Enter"d""d"prompt("Q?", 5), Enter5"5"prompt("Q?", null), Enternull"null"prompt("Q?", undefined), Enterundefined, printsQ? [undefined]"", printsQ?prompt(undefined)undefinedPromptconfirm(undefined)undefined [y/N]Confirm [y/N]alert(undefined)undefined [Enter]toStringthrowsprompt, 20confirm, 20alert), 83 differ from main. 32 are empty-line results. 15 lose the[undefined]label. 35 have the label of anundefinedmessage. 15 have aSymboldefault, where the message no longer prints before theTypeError. A row can be in two groups.alertrow differs.Who depends on
nullfor a bare Enterprompt(q, "")printsQ? [], on main and with this PR. A line fromconsoleis async and is a second reader of stdin.prompt(). Four run the same for""andnull:packages/bun-release/src/console.ts:48and:68,scripts/build/xmac.mjs:1357,test/bake/bake-harness.ts:155.docs/runtime/secrets.mdx. A bare Enter reachedsecrets.set(), which throws fornulland deletes the secret for""(read fromJSSecrets.cpp:159andSecretsLinux.cpp:283, not run). The example now stops on an empty answer.bunmatched 243 with an unqualifiedprompt(,confirm(oralert(call. In those I read theprompt(calls that have??,nullor||on the same line. Most treatnulland""alike (|| "",?? "",?.trim()) or pass a default.nullfor a bare Enter:bun-secrets@0.0.1(?? fail("missing secret value")) and@ax51/bun-component-creator@1.0.1(?? "./").@samyx/preview-stacks@0.28.0reports "stdin closed" fornull, and a bare Enter triggers that on main.prompt(q) || nullgives the old value. Ifnullfor a bare Enter must stay, one alternative breaks nothing: print no[]for an empty default, so thatprompt(q, "")is the clean spelling.Not changed here
alert()andconfirm()read fd 0 one byte at a time.prompt()reads through the 4 KiBBufferedStdin. After a rejected answer,confirm()stops at a CR (mainprompt.rs:140). Soprompt()can read an empty line that nobody typed for it. Without a default that line returnednull, and it now returns"".printf 'n\r\nAlice\n'into[confirm("C?"), prompt("P?")]: main[false,null], this PR[false,""]. Right is[false,"Alice"]. A Windows console line ends in CR LF too (not run here).printf 'a\n\nb\n'into[prompt("1"), alert("2"), prompt("3")]: main["a",null,null], this PR["a",null,""]. Right is["a",null,"b"].null.prompt.length,confirm.lengthandalert.lengthare 1. WebIDL gives 0.EncodedSlice::to_jsstill returns""at its other call sites for bytes that cannot become a string. Throw instead of returning "" when bytes cannot be copied into a JS string #46984 is the change for that class.Other open PRs on these lines
default.unwrap_or(JSValue::NULL)). With the typed result, a stale copy does not compile. So this PR first costs the fewest rewrites.prompt.bind.ts)defaultValueandreturnslines stay. Theb.opaquerespelling of the two is not needed.prompt.rs,web-globals.test.js)prompt.rs:174and:182becomeNoneandSome(default). The fifth answer of itspromptLinesbecomes"".prompt.rs)default.unwrap_or(JSValue::NULL)returns becomeSome(default). Its answer becomesclone_utf8(answer).prompt.rs)default_stringhoist is not needed, because the default converts before the body runs.prompt.rs)prompt()is not needed. Itspromptrow inbuffer-oom.test.tsexpects a throw under the synthetic cap, andclone_utf8does not read that cap.web-globals.test.js)History
CallFrameto schemas and delete#[host_fn],$rustand$newRustFunction#46540 moved that parsing into the schema, so the PR is written again on top of it. It keeps the same three behaviour changes.undefinedmessage is the second commit and has no report. The first commit stands alone (prompt.rsis the same in both). Ask, and I remove the second.A line that cannot become a string
(head -c 2200000000 /dev/zero | tr '\0' a; echo) | bun -e 'prompt()': main returns"", this PR throwsERR_STRING_TOO_LONG. The nextprompt()reads the next line on both.BunString__fromLatin1returns a tagged failure, and the conversion throwsERR_MEMORY_ALLOCATION_FAILED.Measurements (linux x64, release builds of main 449877f and of this diff on it. The
src/diff is the same after the rebase.)prompt(m): 1, 11, 80 bytesprompt(m): CJK 30 bytes, ill-formed 11 bytesprompt(m, "dflt")prompt(m), onprompt(m, "dflt")confirm(m),alert(m)mi_mallocandmi_free, answeredmi_malloc, EnterJSStringcells: answered, Enter, Enter on a string defaultprompt(m),prompt(m, d)write(2):prompt(m),prompt(m, "d"),prompt(m, undefined),prompt(undefined),confirm(undefined)BUN_JSC_useJIT=0, counted withperf_event_open. The number is the slope between 100,000 and 200,000 calls, the minimum of 5 interleaved runs. The spread of the runs is at most 0.4 per call.gdbhit counts onbun-profile, as a slope over 200, 400 and 800 calls, and over 1000 and 2000 calls.heapStats(). Writes:/proc/thread-self/ioover 5000 calls.read(2)is the same on both, 1 per 4096 bytes.nm -S)..textis 82,164,617 -> 82,165,129..rodata,.data,.bssand the size of the stripped file are the same. Host functions are 3 -> 3.GeneratedPrompt.cppis 71 -> 77 lines.perf,valgrindandstraceare not in the container.Tests
test/js/web/web-globals.test.js: 30 new rows, each in a fresh process. The debug build passes all 91 tests. A release build of main fails 14 of the new rows.test/internal/bindgen.test.ts: 226 pass on the debug build.test/integration/bun-types/bun-types.test.ts: 24 pass on a release build. The generated type ofpromptis(message?, defaultValue?) => string | null. On main the result isany.bun run rust:check-all: 12 targets pass. It ran before the rebase onto 98a4365, which did not touch these files.Downsides
prompt(q) ?? fallbackand=== nullno longer catch a bare Enter (prompt(q) || nulldoes). Two npm packages depend on the old value (Notes).ERR_STRING_TOO_LONG. Main returns"".JSStringcell..textgrows by 512 bytes.no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/web/web-globals.test.js