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WalkthroughRuntime generation now receives the output format. IIFE output uses runtime ChangesRuntime and entrypoint lowering
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I reviewed this PR and didn't find any bugs. The fix is surgically scoped and well-tested, but since it changes the bundler's runtime-source selection for --target=node --format=iife (swapping the createRequire flavor for the ambient-require shim) and adjusts what the printer emits for import.meta.main, a maintainer sign-off on those semantics would be worthwhile.
What was reviewed:
- Traced
runtime_require_refin the linker (format != Cjs) against the newauto_polyfill_requiregate (Esm | Iife) — now aligned. - Checked the sole
get_runtime_sourcecaller was updated; node+cjs/node+esm still getRUNTIME_REQUIRE_NODE. - Confirmed the parser's new
!output_format.is_esm()branch is a no-op forCjsandInternalBakeDev(both haveauto_polyfill_requireoff, sorecord_usage_of_runtime_requiredoes nothing). - Verified the printer change leaves
edgecase/ImportMetaMainTargetNode(esm) andedgecase/build-cjs-module#20308(cjs,require_refisNone) byte-identical.
Extended reasoning...
Overview
This PR fixes #24540: --format=iife bundles referencing __require (dynamic require(), require.resolve(), uncalled require, require.main, or the import.meta.main lowering) threw ReferenceError: __require is not defined because the parser only wired up the runtime-part dependency for esm output, while the linker/printer emit __require for every non-cjs format. Four source files change:
src/bundler/ParseTask.rs:auto_polyfill_requirenow matchesEsm | Iife;get_runtime_sourcegains anoutput_formatparameter soTarget::NodewithFormat::Iifefalls through to the ambient-require shim (RUNTIME_REQUIRE_OTHER) instead of thecreateRequire(import.meta.url)flavor, which cannot exist in a script.src/bundler/bundle_v2.rs: sole caller ofget_runtime_sourceupdated to pass the new argument.src/js_parser/p.rs:value_for_import_meta_mainrecords the__requireusage for any non-esm format, so the runtime part isn't tree-shaken in iife output.src/js_printer/lib.rs: fortarget=node, the__require.moduleright operand of theimport.meta.mainlowering is restricted toFormat::Esm; iife (loaded as a CommonJS script) prints the host'smodulelike cjs already did.
Ten new itBundled tests cover node/bun/browser × dynamic require, minified, the no-host-require error message, require.resolve/uncalled/require.main, external require for node, an esm guard, and two import.meta.main iife tests run under both bun and node.
Security risks
None. This is bundler output-shape logic — no auth, crypto, untrusted-input parsing, or FFI/memory safety involved.
Level of scrutiny
Medium-high. The individual edits are small and each is directly justified against the linker's existing format != Cjs rule and esbuild's iife behavior, but they change the runtime shim that ships in every --target=node --format=iife bundle and what the printer emits for import.meta.main. I verified esm and cjs paths are unaffected: node+esm still hits the Target::Node if output_format != Iife arm and keeps createRequire; cjs still has require_ref = None so the printer's new match takes the _ => arm and prints module as before; and the parser's added !is_esm() branch is a no-op for cjs and InternalBakeDev because record_usage_of_runtime_require is gated on auto_polyfill_require, which is off for both.
Other factors
- The tests are strong: they execute the bundle under both bun and node (not just snapshotting output), assert on the presence/absence of
createRequire/import.meta/__require("node:module"), and include a minified variant and a no-host-require negative test. The description reports 9/10 fail on the unfixed build (the esm guard passes on both), confirmed by robobun'sUSE_SYSTEM_BUN=1run. - The PR description is unusually thorough — it traces the mechanism to specific source lines, explains why each
__requireflavor can/cannot appear in an iife, and explicitly scopes out the remaining--target=bun --format=iifeimport.meta.maingap as a separate issue. - I'm deferring rather than approving because the choice of shim for node iife (ambient
requirevs.createRequire) and the__require.module→modulechange for theimport.meta.mainlowering are semantic decisions about emitted code that a bundler maintainer should confirm, even though the reasoning (an iife is a script, soimport/import.meta.urlare unavailable and the hostrequire/moduleare the correct bindings; matches esbuild) looks sound.
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Updated 6:04 PM PT - Aug 20th, 2026
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Actionable comments posted: 2
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Inline comments:
In `@src/js_printer/lib.rs`:
- Around line 3062-3070: Reorder the match arms in the module-printing logic so
the valid commonjs_module_ref case is checked before the Iife or Target::Node
fallback. Preserve the existing literal module output when no valid
commonjs_module_ref exists.
In `@test/bundler/bundler_edgecase.test.ts`:
- Around line 2264-2289: Add a third itBundled case for
ImportMetaMainInvertedMemberTarget that reuses importMetaMainInvertedMemberFiles
and importMetaMainInvertedMemberCapture with target: "node", asserting the
lowered ==/!= printer path for both the member-expression and exponentiation
cases.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
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📒 Files selected for processing (8)
src/bundler/ParseTask.rssrc/bundler/bundle_v2.rssrc/bundler/transpiler.rssrc/js_parser/p.rssrc/js_parser/parse/parse_entry.rssrc/js_printer/lib.rstest/bundler/bundler_cjs.test.tstest/bundler/bundler_edgecase.test.ts
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I reviewed this PR and found no bugs on the current revision — the two precedence issues flagged earlier were fixed in ad9070f and 80ffd46, and the coderabbit test-coverage suggestion landed in a9b0f8a. Because the change spans the parser, printer, and runtime-source selection (notably swapping node+iife to the browser __require shim and the unbound-module-symbol reservation), a human look at the design would still be worthwhile.
What was reviewed:
auto_polyfill_requirenow matches the linker'sformat != cjsrule;get_runtime_sourcecallers all updated.- Printer's module-operand match preserves pre-PR cjs+node behavior (literal
module, notcommonjs_module_ref) and only emits__require.modulefor esm. - Both
EImportMetaMainprinter arms now wrap at the right precedence and the inverted form is enumerated in the BinPow left-operand list alongsideEUndefined/minifiedEBoolean. compute_initial_reserved_namesreservesmoduleonly for cjs, so the per-file unbound symbol is what makes the chunk-wide renamer avoid the name for iife — verified by theShadowedModuleBindingtest.
Extended reasoning...
Overview
The PR fixes #24540 (ReferenceError: __require is not defined in --format=iife output) by bringing the parser's auto_polyfill_require gate in line with the linker's runtime_require_ref rule (both now cover esm and iife), and by threading output_format into get_runtime_source so node+iife uses the browser-style shim (an iife cannot contain import { createRequire } or import.meta.url). It also extends import.meta.main lowering to iife: renames lower_import_meta_main_for_node_js → lower_import_meta_main, records a __require use so the runtime part survives tree-shaking, declares an unbound module symbol so the renamer keeps user bindings of that name away from the host's module, and reworks the printer's EImportMetaMain arm to (a) keep import.meta.main for iife+bun, (b) parenthesize both the kept !import.meta.main and the lowered ==/!= forms at the right precedence, and (c) print the literal module operand for iife/cjs vs __require.module for esm. Eight files touched; ~350 diff lines, roughly half tests.
Security risks
None identified. This is bundler codegen; no untrusted input parsing beyond what the parser already handles, no auth/crypto/permissions.
Level of scrutiny
Medium-high. The parser/printer/linker are core bundler paths where a subtle mistake miscompiles user code silently. The two precedence bugs my earlier review caught (unwrapped !import.meta.main under a member target, and left of **) illustrate that; both were fixed with regression tests. On the current revision the bug hunter found nothing, and I traced the printer's new match arms against the pre-PR behavior for every format×target combination that reaches the lowering branch — cjs+node still prints the literal host module (the coderabbit thread confirms that ordering is deliberate), esm+node still prints __require.module, and iife now prints the host module with the name reserved via the new unbound symbol.
Other factors
Test coverage is thorough: 8 new bundler_cjs cases (dynamic require × three targets, minified, no-host-require error, require.resolve/uncalled/.main, external require, esm guard) and 8 new bundler_edgecase cases (iife × three targets run under both bun and node, required-as-non-main, inverted forms, shadowed module, member/** precedence for both keep and lower branches). All comment-cop and reviewer threads are resolved. What tips this to defer rather than approve is the scope: it changes which runtime shim is compiled into node+iife bundles and introduces a per-file unbound-symbol pattern to reserve a name chunk-wide — both are reasonable, but they're design calls a bundler maintainer should sign off on rather than land purely on automated review.
The printer rewrites require(expr), require.resolve(expr), uncalled require references and the import.meta.main lowering to the runtime's __require in every output format except cjs, but the parser only imported the runtime part defining it for esm. In iife output the part was tree-shaken and the bundle threw "__require is not defined". Enable the runtime require import for iife as well. For target node the iife runtime uses the same require shim as the browser, since an iife can neither import node:module nor read import.meta.url; this also fixes external require() calls in node iife output, whose runtime part was printed as a reference to itself. import.meta.main records the __require use in iife output and, for node, compares against the host module binding instead of __require.module, which only exists as the esm lowering. Fixes #24540
Decide in one place (ParseTask) which entry points get import.meta.main lowered: esm output for node and iife output for every target but bun, whose `// @bun` iife is loaded as an ES module where import.meta.main works and no `module` binding exists. The printer keeps it verbatim for that combination instead of emitting a comparison that throws. When the iife lowering prints the host's `module`, the parser declares an unbound `module` in the entry point's scope so the renamer keeps bundled bindings off that name, as cjs output already does through its reserved names; otherwise a top-level `module` binding anywhere in the bundle silently made import.meta.main false. The lowering prints as an == expression, so parenthesize it like one: `!import.meta.main` was printed as `!x.main == module` (also in the existing esm lowering for node). Tests cover all three iife targets as main and when required from another module, the inverted forms, a shadowing `module` binding and the esm/node parenthesization.
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I re-reviewed after the fixes for the two precedence issues I raised earlier (both addressed in ad9070f / 80ffd46) and CodeRabbit's test-coverage note (addressed in a9b0f8a); this pass found no further bugs. Given the scope — runtime-source selection now varies by output format, the EImportMetaMain printer arm is restructured with new precedence wrapping, and the parser declares an unbound module symbol to steer the iife renamer — a human look is still worthwhile.
What was reviewed:
auto_polyfill_require/runtime_require_refnow agree for iife; esm and cjs paths unchanged (guarded by the new__require_esm_dynamic_require_target_nodetest).- The node-iife →
RUNTIME_REQUIRE_OTHERshim swap: verified it can't leak into esm/cjs (theFormat::Iifeguard is the only new arm) and that bun-iife keepsimport.meta.require. - The
modulename-reservation viadeclare_generated_symbol(Unbound, "module"): only declared once per file, and theImportMetaMainIIFEShadowedModuleBindingtest confirms sibling files'modulebindings are renamed too. - Both new precedence wraps (
gte(Prefix)for the kept inverted form,gte(Equals)for the lowered form) plus theBinPowleft-operand list entry, with tests covering member-target and**for keep and lowered paths.
Extended reasoning...
Overview
This PR fixes #24540: --format=iife bundles that print the runtime's __require (dynamic require(), require.resolve, uncalled require, require.main, or a lowered import.meta.main) previously threw ReferenceError: __require is not defined because the parser's auto_polyfill_require feature was only enabled for esm output while the linker's runtime_require_ref prints __require for both esm and iife. The fix has four coordinated pieces across five source files: (1) ParseTask.rs enables auto_polyfill_require for iife and threads output_format into get_runtime_source so node-iife uses the ambient-require shim instead of the createRequire(import.meta.url) flavor that can't exist in a script; (2) p.rs records the __require runtime use whenever import.meta.main will be lowered and, for iife, declares an unbound module symbol so the renamer keeps bundled module bindings away from the host's; (3) lib.rs restructures the EImportMetaMain printer arm to keep import.meta.main for bun-iife (loaded as ESM), lower it elsewhere with the correct right-hand operand per format, and parenthesize both the kept-inverted and lowered forms at their precedence levels (including the BinPow left-operand special case); (4) bundle_v2.rs and transpiler.rs are mechanical signature updates. Test coverage is thorough: 8 new bundler_cjs cases and 8 new bundler_edgecase cases exercising all three targets, minified output, the no-host-require error path, all four require reference forms, external requires, an esm regression guard, and every precedence position touched.
Security risks
None. This is bundler output-generation logic with no auth, crypto, filesystem, or network surface. The change affects what JavaScript text the bundler emits, not how it processes untrusted input.
Level of scrutiny
High. The bundler's parser/printer/linker coordination is one of Bun's most correctness-sensitive subsystems — a mis-emitted token is a miscompile that silently breaks user code at runtime. This PR crosses four of those layers with invariants that must agree by construction ("must match runtime_require_ref", "must agree with EImportMetaMain in the printer"), and the review history shows the precedence handling needed two follow-up fixes to close the class. The design decision to route node-iife through the browser shim (rather than, say, erroring or emitting a top-level createRequire outside the iife) is defensible and matches esbuild, but is the kind of behavioral choice a maintainer should sign off on.
Other factors
All prior review threads are resolved: my two precedence findings (member-expression target and ** left operand for the kept-inverted form) were fixed with tests; CodeRabbit's request for a lowered-form variant of those tests was addressed; CodeRabbit's commonjs_module_ref ordering concern was withdrawn after the author explained the host-module semantics. The bug-hunting system found nothing this run. The declare_generated_symbol(Unbound, "module") mechanism is a new pattern in the parser (mirroring what compute_initial_reserved_names does for cjs) and is worth a maintainer's eye to confirm it's the right layer for name reservation, though the ImportMetaMainIIFEShadowedModuleBinding test proves it works end-to-end including for sibling files.
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Fixes #24540
Problem
--format=iifebundle whose code contains arequire()with a non-literal argument (alsorequire.resolve(expr), an uncalledrequirereference, orrequire.main) throws on load:ReferenceError: __require is not defined. Same for every target (browser,node,bun); the same input works with--format=esm.__requirefor every output format exceptcjs(runtime_require_refinsrc/bundler/linker_context/postProcessJSChunk.rs:86andgenerateCompileResultForJSChunk.rs:115), but the parser only imports the runtime part that defines__requireforesm(auto_polyfill_require,src/bundler/ParseTask.rs). Iniifeoutput nothing references the part, so it is tree-shaken away.--target=node --format=iifeis broken even for a plain externalrequire("pkg"), which does pull the part in through the linker: the node flavor of the part isimport { createRequire } from "node:module"; ... createRequire(import.meta.url), and in an iife that import is itself printed asvar import_node_module = __require("node:module"), so the output fails withTypeError: __require is not a function.import.meta.main/require.main === modulein an iife entry point hit the same missing binding: the lowering prints__require.main == module(__require.main == __require.modulefor node) without recording a use of__require.Fix
src/bundler/ParseTask.rs:auto_polyfill_requireis enabled foriifeas well asesm, so a file using any of the rewritten forms imports the runtime__requirepart, the same way it already does in esm output. This is the parser-side counterpart of the linker's existingformat != cjsrule; the two were out of sync.src/bundler/ParseTask.rs,src/bundler/bundle_v2.rs: the runtime source is chosen by output format too.--target=node --format=iifeuses the same shim as the browser flavor (the one esbuild emits for iife on every platform): an iife can contain neither animportnorimport.meta.url, and node loads it as a CommonJS script, where the shim finds the hostrequireand forwards to it (require.resolve,require.mainand relative paths all resolve against the bundle file). Without a hostrequireit throws esbuild'sDynamic require of "x" is not supported.esmandcjsoutput for node are unchanged;--target=bunkeepsimport.meta.requirefor iife, since Bun loads the// @bunoutput as an ES module where that is the only require available.src/js_printer/lib.rs: the printer keepsimport.meta.mainas is for--target=buniife output (Bun loads it as an ES module, where it works and nomodulebinding exists) and lowers it everywhere else, matching the parser (lower_import_meta_maininParseTask.rs). The__require.moduleoperand is only printed for esm output, where there is nomodulebinding andcreateRequire()'s.moduleisundefinedon both sides of the==; an iife runs as CommonJS, so there the host'smoduleis the right operand, the same as cjs output prints. Both the lowered comparison and the kept inverted form (!import.meta.main) are parenthesized where precedence requires it: as a member-expression target ((!import.meta.main).toString()) and as the left operand of**(where an unparenthesized!prefix is a SyntaxError). esm and cjs output are byte for byte unchanged.src/js_parser/p.rs:value_for_import_meta_mainrecords the__requireuse whenever the printer will lower, and for iife output declares an unboundmodulesymbol so the renamer keeps bundled bindings of that name away from the host'smodulethe lowering compares against (cjs output already reserves the name the same way).test/bundler/bundler_cjs.test.ts(cjs/__require_iife_*: dynamic require for node, bun and browser targets, minified, the no-host-require error,require.resolve/uncalledrequire/require.main, external require for node, plus an esm guard) andtest/bundler/bundler_edgecase.test.ts(theImportMetaMain*cases: iife for all three targets run under both bun and node, also when required as a non-main module, the inverted forms, a shadowedmodulebinding, and the inverted form as a member-expression target). All but the esm guard fail on the unfixed build.test/bundler/esbuild/{default,dce,loader,importstar,splitting,extra}.test.ts,test/bundler/transpiler/react-compiler.test.ts,test/bundler/bun-build-api.test.tsand the two modified files pass on a debug build; esm and cjs output for the repro are identical before and after.import.meta.*(other thanmain) is printed verbatim in iife output; that is separate from the missing__requirebinding. bundler: declare require inside a CommonJS wrapper whose body contains direct eval #35581 restricts itself to esm because of this bug and can be extended to iife once this lands.Background
src/runtime.jsplus a per-target__requiredefinition inParseTask.rs) to every bundle. Each helper is its own part and is only kept in the output when some file's part depends on it; otherwise tree shaking removes it.auto_polyfill_requireis a parser feature: when set, arequirethat cannot be bundled makes the file import__requirefrom the runtime, creating that dependency. The printer, independently, decides what name to print for such arequire(require_ref): the runtime's__requirein esm/iife, plainrequirein cjs.__requireflavors:import.meta.requirefor bun,createRequire(import.meta.url)for node, and esbuild's shim (use the ambientrequireif there is one, otherwise throwDynamic require of ... is not supported) for everything else. The first two need module syntax, which is why they cannot appear in an iife for node.import.meta.mainhas no equivalent outside esm, so the bundler lowers it to arequire.main == modulecomparison; entry points are the only files where it is not folded to a constant.Repro (released bun)
External require on node, before this change (the runtime part is present but defines itself through itself):
After: all three targets print
function; esm and cjs output is unchanged.no test proof · iteration 0 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/bundler/bundler_edgecase.test.ts