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Status: fix is ready for review in this PR (#42752). How I reproduced it (bun 1.4.3 canary 09bb546 and // main.ts
import "./a.ts";
// a.ts
import "./b.ts";
export function fa() { return [require("./c.ts").c, __dirname, __filename]; }
console.log("a done");
// b.ts
import { fa } from "./a.ts";
console.log("b calls fa ->", fa());
// c.ts
export const c = "c";With this branch: Test: |
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Inline comments:
In `@src/jsc/bindings/ImportMetaObject.cpp`:
- Around line 729-731: Replace the source-text prefix check in the import-meta
linking logic with explicit hoisted-binding metadata carried by the source
provider or module record. Only strip declarations and perform link-time
initialization when the corresponding hoisted-binding bit is set, preserving
undefined-before-initializer behavior for ordinary user source.
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src/ast/ast_result.rssrc/js_parser/p.rssrc/js_parser/parse/parse_entry.rssrc/js_printer/lib.rssrc/jsc/RuntimeTranspilerCache.rssrc/jsc/bindings/ImportMetaObject.cppsrc/jsc/bindings/ImportMetaObject.hsrc/jsc/bindings/ZigGlobalObject.cpptest/js/bun/resolve/import-meta.test.js
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Updated 4:19 PM PT - Sep 15th, 2026
✅ @robobun, your commit 80b9968212ab493e3ec66309d719badcbf1a11d3 passed in 🧪 To try this PR locally: bunx bun-pr 42752That installs a local version of the PR into your bun-42752 --bun |
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About the "whitespace-minified modules with a user-defined The hook matches the output of the runtime transpiler, not the file on disk. The runtime transpiler always prints with spaces (the minify flags exist only for // a.mjs, one line
var __dirname=import.meta.dir;var {require}=import.meta;import"./b.mjs";export function f(){return[typeof __dirname,typeof require]}console.log("a body",f());
// b.mjs
import{f}from"./a.mjs";console.log("early",f());reaches JSC as: var __dirname = import.meta.dir;
var { require } = import.meta;
import"./b.mjs";Output with this branch and with bun 1.4.3 is the same: So a variable that the user declared keeps no value before the module body. The test in d66c0ad pins this for |
…hen the module is linked The runtime transpiler declares these three names as var at the start of an ES module. A var has no value until the module body starts. A function declaration of the module is callable before that, through an import cycle, and read them as undefined (require: TypeError). js_printer now owns one table, HOISTED_MODULE_BINDINGS, with the exact declaration, the variable and the import.meta property of each name. print_ast prints the declarations from it at the very start of the output. The parser no longer adds its own part for __dirname and __filename at runtime, which had no fixed position. JSC creates import.meta while it links the module, after it created the module environment. ImportMetaObject::initializeHoistedBindings reads the same table through Bun__hoistedModuleBinding, finds the declarations at the start of the source, and stores the values in the environment. The printed text for __dirname and __filename changes, so the transpiler cache version goes from 33 to 34.
The runtime printer never prints user code without spaces, so a declaration that the user wrote cannot equal the text of a hoisted declaration. This case pins that.
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Each declaration, variable and import.meta property is decided by the binding itself, so they are methods on the enum and not stored fields.
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Inline comments:
In `@src/js_printer/lib.rs`:
- Around line 8006-8007: Restrict the HoistedModuleBinding::Dirname and
HoistedModuleBinding::Filename cases to emit their references only when
printer.options.target is bun_ast::Target::Bun; preserve the existing
tree.uses_dirname_ref and tree.uses_filename_ref behavior for Bun targets and
avoid emitting these Bun-specific properties for Browser or Node ESM output.
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src/js_parser/p.rssrc/js_printer/lib.rssrc/jsc/bindings/ImportMetaObject.h
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Add a common string for these
initializeHoistedBindings made an Identifier from a C string for each variable and each import.meta property, on each module link. The variable names are now common strings, made one time per VM. The values come from the LazyProperty fields that the import.meta getters read, so no property name is necessary. Rust now exports only the declaration text.
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Done in 4d4e1e0. I read "these" as the names that
If you meant other strings, tell me which ones. |
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Problem
require,__dirnameor__filenamegetsundefinedwhen another module calls it through an import cycle, before its module body starts. Forrequire:TypeError: require is not a function. (In 'require("./c.ts")', 'require' is undefined). Regression in 1.1.43 (transpiler: dont inlineimport.meta.require#16222).var(print_ast,src/js_printer/lib.rs). Avarhas no value until the body starts.import.meta.requireat each call site is not an option: vite runsnew Function(fn.toString())(bunx --bun vite build error during build: [vite:css] import.meta is only valid inside modules. #15738).Fix
js_printerowns the declaration text (theHoistedModuleBindingenum).print_astprints all three declarations at the start of the output (the parser part for__dirname/__filenameis gone). Transpiler cache version: 33 to 34.ImportMetaObject::initializeHoistedBindingsmatches that text (read through FFI) at the start of the source and stores the values in the module environment. The variable names are common strings.test/js/bun/resolve/import-meta.test.js. Both fail on bun 1.4.3. Not fixed here:import ... from "bun"(Hoist the lowering ofimport ... from "bun"above the module body at runtime #39206),requirein macros (Macro error in Bun 1.1.43 #16311).Background
import.meta, and sets eachvartoundefined.aimportsb,bimportsa),bruns first and can call a function declaration ofa.Notes
No issue is filed for this. It was found by a comparison of bundled and unbundled evaluation order (
test/bundler/splitting-fuzz.ts, generator seed 9881).Repro (bun 1.4.3 canary 09bb546 and main, linux x64, same with
.js):Before:
TypeError: require is not a function(andundefinedfor the two paths whenrequireis not used). After:b calls fa -> [ "c", "/tmp/repro", "/tmp/repro/a.ts" ], thena done.bun build --target=bunof the same graph was not affected.Printed output at runtime, before and after:
The bundle-time path (
bun build,bun build --no-bundle,Bun.Transpiler), which prints string literals for__dirname/__filename, is not changed. Its output is byte-identical to 1.4.3.Same class, not fixed here:
import { x } from "bun"printsvar {x} = globalThis.Bun;. It has noimport.meta, so JSC does not call this hook for it. The fix there is to stop the lowering and link the nativebunmodule, which is the decision in Hoist the lowering ofimport ... from "bun"above the module body at runtime #39206 and Stop rewriting a literal import("bun") to Promise.resolve(globalThis.Bun) #37730.requirein a macro module:print_astdeclaresrequireonly fortarget == Bun, and a macro has the macro target, sorequireis not defined there at all. This is the open regression Macro error in Bun 1.1.43 #16311 (also from transpiler: dont inlineimport.meta.require#16222). It is a one-line change of that condition and needs its own test.__dirname/__filenamein macros work as before.Why the signal is in the text, and not a flag from Rust: at link time the hook has a
JSModuleRecord, whose source provider is aJSC::SourceProvider*. Bun builds without RTTI, and only a provider with source typeBunTranspiledModule(it hasmodule_info, which onlybun test --isolate/--parallelproduce) can be downcast toZig::SourceProvidersafely. An ordinarybun runmodule has source typeModule, the same as JSC's own providers (anode:vmSourceTextModulein the main context reaches this hook too). A flag also needs a new field in the transpiler cache metadata and in about 8ResolvedSourceproducers. The declaration text is defined once, in Rust, and C++ reads it throughBun__hoistedModuleBindingDeclaration. C++ has a table in the same order with the variable name (a common string) and theLazyPropertythat holds the value. A debug assertion checks that each declaration names its variable, and the tests check each value. The printer has adebug_assert!that the declarations start the output, and the test module starts with a"use client"directive, so a later change that prints something before them fails CI. If a maintainer prefers the flag, the C++ side and the tests stay as they are.Other alternatives that I rejected:
function require() {}stub that forwards toimport.meta.require. It has norequire.resolve,require.cacheorrequire.mainbefore the body runs, andrequire !== import.meta.require.import { require } from ...per module. It costs a module record for each file that usesrequire.varwith one of these names. That changes avar __dirname = ...that the user wrote (common in ESM output of esbuild), which must readundefinedbefore its initializer runs.#42590 changes the same hook to call
setModuleGraphon the newimport.meta.initializeHoistedBindingsis the last call in the hook because it createsimport.meta.require, sosetModuleGraphgoes before it when the two PRs meet.If the source does not start with the declarations, nothing changes: each
vargets its value when the body starts, as before.Other checks on the debug build:
require,require.resolve,require.cache,require.mainandrequire === import.meta.requiregive the same results before and after the body starts. Stack trace line and column numbers are the same as on 1.4.3.__dirnameor only__filename, a user-declaredvar require/var __dirname(stillundefinedearly), CommonJS, the entry point in a cycle,export default function, async and generator function declarations, a Worker,bun test,bun test --isolate(module record built frommodule_info),--inspect(all module variables captured), a transpiler cache restore,BUN_JSC_validateExceptionChecks=1.import-meta,function-tostring-require,runtime-transpiler,transpiler.test.js,require-esm-evaluating-cycle,dynamic-import-tla-cycle,transpiler-cache,require.test.ts,run-eval,run-cjs,debugger-buntranspiledmodule,isolation,esModule.test/cli/run/require-cache.test.ts: the leak tests time out on this debug build (100000 iterations in 60 s). Their fixtures are CommonJS or declare their ownrequire, so they do not reach the changed code.[human-review] gate passed · iteration 1 · 10 files touched
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