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The printer writes the members of the classic JSX factory and fragment
verbatim. Only the tsconfig.json loader checked that they are
identifiers. Bun.build({ jsx }), --jsx-factory, --jsx-fragment and the
bunfig.toml jsxFactory and jsxFragment keys accepted any text, so a value
such as "h;alert(1);h" produced output that does not parse while the
build reported success.
Pragma::invalid_member holds the rule. The tsconfig.json loader calls it
for its warning. MemberListOption::check calls it for the three option
sites, which now report an error. The first member cannot be a reserved
word, with the exceptions esbuild makes (null, this, import.meta). A
fragment can still be a constant that the printer can write as it is,
for example '['.
The @jsx and @jsxFrag pragmas do not change.
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Status: ready for a maintainer. The fix and the tests are pushed. All review threads are resolved. Reproduction on bun 1.4.3 (release build): // repro.mjs
import { mkdirSync, writeFileSync, readFileSync } from "node:fs";
mkdirSync("f", { recursive: true });
writeFileSync("f/a.jsx", "export const x = <div a='1'><></></div>;\n");
const r = await Bun.build({
entrypoints: ["f/a.jsx"],
outdir: "f/out",
jsx: { runtime: "classic", factory: "h;alert(1);h", fragment: "F" },
});
console.log(r.success, readFileSync("f/out/a.js", "utf8").split("\n")[1]);
// true var x = /* @__PURE__ */ h;alert(1);h("div", {
With this branch,
CI.
Each of these passed in the other run. Both are reported to the maintainers of those tests. The other entries in the annotations passed on a retry. |
WalkthroughChangesJSX option validation
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🟡 Moderate · up to Malformed JSX configuration values can silently produce a different factory or fragment expression instead of the promised configuration error. Reject empty member segments before merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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I reviewed this PR and didn't find any bugs. Because it changes user-facing behavior across four option surfaces (previously-tolerated inputs like ".", (0,h), a-b now hard-error) and makes a design call to accept only the subset of esbuild's constant fragments that survive Bun's current member-list printing, a human sign-off on those choices is worthwhile.
What was reviewed:
Pragma::invalid_memberandis_verbatim_constant: traced tokenization edge cases (".","",import.meta,null, lone','', unclosed'a", non-ASCII) against the accepted/rejected test vectors — all consistent.- Confirmed all four entry points (JSBundler, CLI, bunfig, tsconfig) route through the shared helper; error text quotes the value via
bun_core::fmt::quote, goes to stderr, and exits nonzero / throwsTypeError. - Tests: subprocess tests drain pipes via
Promise.all, assert combined{stdout, stderr, exitCode}, usetempDir/bunEnv/test.concurrent, and cover the factory×fragment × build/run/compile × CLI/API/bunfig/tsconfig matrix.
Extended reasoning...
Overview
This PR adds input validation for the JSX classic-transform factory and fragment options so that non-member-expression strings are rejected at parse time instead of being emitted verbatim into bundled output. A shared Pragma::invalid_member predicate in src/options_types/jsx.rs (dotted-identifier chain, first member not a reserved word, esbuild's null/this/import.meta exceptions) is called from four surfaces: Bun.build({ jsx }) (throws TypeError), --jsx-factory/--jsx-fragment (stderr + exit 1), bunfig jsxFactory/jsxFragment (parse error), and tsconfig (existing warning path, now via the shared helper so reserved-word first members also warn). A MemberListOption enum carries per-option error text, and is_verbatim_constant lets the fragment accept a limited set of literal scalars ('[', true, 0, non-zero-leading integers, quoted strings without ./\/newlines) that already print correctly today. ~200 lines of tests in test/bundler/bundler_jsx.test.ts cover valid/invalid/constant inputs across all four surfaces plus --compile.
Security risks
The change is a hardening: it closes a fuzzer-found path where a config-supplied string was written verbatim into JS output (h;alert(1);h("div", …)). No new attack surface is introduced — the only new behavior is rejection with an error. Error messages echo the rejected value via bun_core::fmt::quote, which is the repo's sanctioned quoter. No auth, crypto, filesystem, or network paths are touched.
Level of scrutiny
Medium. The core validator is ~40 lines of pure-function string logic using the sanctioned bun_core::strings::tokenize/index_of_any helpers, and the four call-site wirings each mirror the neighboring option-handling pattern in their file. What raises the bar above "approve" is not correctness risk but product/design surface: (1) inputs that previously produced parseable-by-accident output ((0,h), a-b) or were silently ignored (".") now hard-error, which is a deliberate compat break the PR justifies against esbuild and a code search but a maintainer should confirm; (2) the constant-fragment acceptance is intentionally narrower than esbuild's full JSON-scalar support because Bun lacks the AST node — accepting only "what prints intact today" is a reasonable interim, but it's a design decision worth a human nod; (3) the tsconfig path now warns on reserved-word factories where it previously did not.
Other factors
The PR follows REVIEW.md closely: the fix is at the shared helper (one source of truth), every sibling entry point is covered in the same PR, error messages name the option / quote the value / state the constraint with an example, and the test suite hits the full variant matrix including the --compile-writes-nothing check. Subprocess tests use the required Promise.all drain pattern and combined-object assertions. The PR body documents the intentional exclusion of @ jsx/@ jsxFrag pragmas (blocked on a lexer line-ending issue) and the relationship to open PR #40857's define-value predicate. No CODEOWNERS entries cover the changed files. No prior review threads exist on the PR.
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Updated 11:15 PM PT - Sep 13th, 2026
❌ @robobun, your commit 39dfac5 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 42676That installs a local version of the PR into your bun-42676 --bun |
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Actionable comments posted: 1
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Inline comments:
In `@src/options_types/jsx.rs`:
- Line 253: Update invalid_member to reject empty member segments before calling
strings::tokenize: detect leading, trailing, or consecutive '.' characters and
return None for those inputs. Preserve existing validation for member
expressions with non-empty segments.
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src/options_types/jsx.rs
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Problem
Bun.build({ jsx: { factory: "h;alert(1);h" } })reportssuccess: trueand writesvar x = /* @__PURE__ */ h;alert(1);h("div", {, which does not parse.--jsx-factory,--jsx-fragmentand the bunfig.toml keys do the same.Pragma::member_list_to_components_if_different(src/options_types/jsx.rs) splits the text on.and the printer writes each piece verbatim. Only the tsconfig.json loader checks that the pieces are identifiers.Fix
Pragma::invalid_memberholds the rule: each member is an identifier, and the first is not a reserved word (esbuild's exceptions:null,this,import.meta). The tsconfig.json loader calls it for its warning.MemberListOption::checkapplies it where an option is read.Bun.buildthrows aTypeError. The CLI flags and bunfig.toml exit 1 with an error, as esbuild does.'[',true,123), as in esbuild."."was ignored without a message (jsx: keep the default factory when a tsconfig or pragma factory has no members #39631) and is now an error. An empty string still means unset.test/bundler/bundler_jsx.test.ts. 54 of the 85 new tests fail on bun 1.4.3.Background
<a/>intofactory("a", null)and<></>intofactory(fragment, null).jsx::Pragmastores both as aMemberList, for example["React", "createElement"].Bun.build, the CLI flags, bunfig.toml, and the@jsx/@jsxFragpragmas. The pragmas do not change.Notes
Pragmas.
@jsx h;alert(1);hprints the same text, but the file author writes both the pragma and the code. The pragmas cannot use this rule yet. The lexer includes the line end in the argument of a pragma inside a block comment, so the docblocksets the factory to
"h\n". That printsh\n("a", null), which works. The rule would reject it and fall back toReact.createElement. That needs the lexer to end the argument at the line end first (the pragma spans are also what #39636 touches).Constant fragments. esbuild takes any JSON scalar for the fragment, for example
--jsx-fragment='"["'for Mithril. Bun has no AST node for one (default/JSXConstantFragmentsintest/bundler/esbuild/default.test.tsis disabled). The text becomes a member and the printer writes it as it is, so'[',"[",'',true,false,0and123give correct output on 1.4.3.checkaccepts exactly the constants that come through that path intact: no.(members are split on it,'a.b'printed'a['b']) and no\(nothing validates an escape). Other constants were broken output before and are an error now.Tests. The 31 new tests that pass on 1.4.3 cover input that must keep working: valid member expressions, constant fragments, the empty string.
Text that parsed by accident.
(0,h)anda-bgave output that parses. They are not member expressions. esbuild rejects them too.".". #39631 left--jsx-factory=.andBun.build({ jsx: { factory: "." } })as they were: ignored without a message (#39631 (comment)). This PR makes them an error, like esbuild.tsconfig.json. The loader keeps its warning text. A reserved word as the first member (
"jsxFactory": "if") now warns and keeps the default. Before, it printedif("a", null).Splitting. The text is split with
tokenize, as before."h."meansh. The open #40857 addsis_member_chain_define_valueinsrc/bundler/defines.rsfor define values, which rejectsh.anda..b. The PR that lands second can share one predicate.The
TypeErrorhas the codeERR_INVALID_ARG_TYPEfromthrow_invalid_arguments, like thejsx.runtimeerror next to it.A GitHub code search found 40 bunfig.toml files that set
jsxFragment. All use a dotted identifier (Fragment,React.Fragment,v.fragment). Thebun build --jsx-fragmentuses in package.json scripts areFragmentandReact.Fragment.Suites run with the debug build:
test/bundler/bundler_jsx.test.ts(142 pass, 5 todo),test/bundler/esbuild/tsconfig.test.ts,test/bundler/transpiler/jsx-tsconfig-react-jsx.test.ts,test/bundler/transpiler/jsx-production.test.ts,test/js/bun/transpiler/transpiler-tsconfig-uaf.test.ts,test/bundler/bundler_files.test.ts,test/cli/run/jsx-namespaced-attributes.test.ts,test/cli/run/jsx-symbol-collision.test.ts,test/internal/source-lints/.cargo clippyonbun_options_types,bun_resolver,bun_bunfigandbun_runtimereports nothing.Self-reviewed: the review found that an earlier version also applied the rule to the pragmas and broke the docblock form above, and that it rejected constant fragments. Both are addressed: the pragmas do not change, and constants are accepted.
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