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fix(bundler): only use Node ESM semantics for __toESM when target is Node.js

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The isNodeMode flag for __toESM should only be set when BOTH:
1. The target is Node.js (not browser/bun)
2. The importing file is in Node ESM mode (.mjs/.mts or "type": "module")

For browser/bun targets, __esModule markers should ALWAYS be respected,
regardless of the file's module type. This fixes the issue where packages
with "type": "module" were incorrectly getting Node ESM semantics even
when building for browsers.

This matches both:
- Babel/TypeScript interop expectations for browser bundles
- Node.js native ESM behavior when targeting Node.js

Fixes the issue reported by the original issue author where their React
app with "type": "module" was still getting double-wrapped default exports.

https://claude.ai/code/session_01AsubwvFrQCVbLi4YRhoMVf
This commit is contained in:
Claude
2026-02-12 09:11:47 +00:00
parent cbe91010fb
commit 0777ba1dee
2 changed files with 115 additions and 40 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
src/js_printer.zig
View File

@@ -1771,7 +1771,10 @@ fn NewPrinter(
}
if (wrap_with_to_esm) {
if (p.options.input_module_type == .esm) {
// Only use Node ESM semantics (ignore __esModule) when targeting Node.js
// AND the importing file is in Node ESM mode (.mjs/.mts or "type": "module").
// For browser/bun targets, always respect __esModule markers.
if (p.options.target == .node and p.options.input_module_type == .esm) {
p.print(",");
p.printSpace();
p.print("1");
@@ -1872,7 +1875,10 @@ fn NewPrinter(
p.print(".then((m)=>");
p.printSymbol(p.options.to_esm_ref);
p.print("(m.default");
if (p.options.input_module_type == .esm) {
// Only use Node ESM semantics (ignore __esModule) when targeting Node.js
// AND the importing file is in Node ESM mode (.mjs/.mts or "type": "module").
// For browser/bun targets, always respect __esModule markers.
if (p.options.target == .node and p.options.input_module_type == .esm) {
p.print(",1");
}
p.print("))");

145
test/bundler/bundler_cjs.test.ts
View File

@@ -6,23 +6,25 @@ import { itBundled } from "./expectBundled";
// The key insight from the code change:
// - `input_module_type` is set based on the RESOLVER's module type determination
// (file extension .mjs/.mts and package.json "type" field), NOT on syntax detection.
// - When the importing file is in Node ESM mode (.mjs/.mts or "type": "module"), isNodeMode = 1
// - When the importing file is NOT in Node ESM mode (regular .js), isNodeMode = 0
// - The isNodeMode flag is ONLY set when BOTH conditions are true:
// 1. The target is Node.js (not browser/bun)
// 2. The importing file is in Node ESM mode (.mjs/.mts or "type": "module")
//
// This means:
// - Regular .js files RESPECT __esModule markers and extract exports.default
// - .mjs files IGNORE __esModule markers and wrap the entire module as default
// - For browser/bun targets: __esModule is ALWAYS respected
// - For Node.js target with Node ESM files: __esModule is ignored (matches Node.js behavior)
// - For Node.js target with regular .js files: __esModule is respected
//
// This matches the correct Node.js behavior where __esModule is a Babel/TypeScript
// interop convention that only applies in non-Node-ESM contexts.
// This matches both Babel/TypeScript interop expectations for browser bundles
// and Node.js native ESM behavior when targeting Node.js.
describe("bundler", () => {
// ============================================================================
// Tests with regular .js entry file (NOT Node ESM mode)
// These use isNodeMode=0, which RESPECTS __esModule
// Tests with regular .js entry file (default browser target)
// __esModule is ALWAYS respected for browser targets
// ============================================================================
// Test 1: import with __esModule marker - RESPECTED in .js files
// Test 1: import with __esModule marker - RESPECTED
itBundled("cjs/__toESM_import_syntax_with_esModule", {
files: {
"/entry.js": /* js */ `
@@ -36,7 +38,7 @@ describe("bundler", () => {
`,
},
run: {
// With .js file (NOT Node ESM), isNodeMode=0, so __esModule IS RESPECTED
// Browser target: __esModule IS RESPECTED
// The default import gets exports.default
stdout: '{"value":"default export"}',
},
@@ -150,10 +152,10 @@ describe("bundler", () => {
// ============================================================================
// Tests with different targets
// Target doesn't affect isNodeMode - it's based on resolver module type
// Only Node.js target + Node ESM files ignore __esModule
// ============================================================================
// Test 7: target=node
// Test 7: target=node with .js entry - __esModule RESPECTED
itBundled("cjs/__toESM_target_node", {
files: {
"/entry.js": /* js */ `
@@ -167,11 +169,13 @@ describe("bundler", () => {
},
target: "node",
run: {
// .js file is NOT Node ESM, so __esModule would be respected if present
// No __esModule here, so entire module wrapped as default
stdout: '{"x":1,"y":2}',
},
});
// Test 8: target=browser
// Test 8: target=browser - __esModule ALWAYS RESPECTED
itBundled("cjs/__toESM_target_browser", {
files: {
"/entry.js": /* js */ `
@@ -189,7 +193,7 @@ describe("bundler", () => {
},
});
// Test 9: target=bun
// Test 9: target=bun - __esModule ALWAYS RESPECTED
itBundled("cjs/__toESM_target_bun", {
files: {
"/entry.js": /* js */ `
@@ -209,10 +213,10 @@ describe("bundler", () => {
// ============================================================================
// Tests with different output formats
// Output format doesn't affect isNodeMode either
// Output format doesn't affect isNodeMode
// ============================================================================
// Test 10: format=esm with __esModule (should be respected in .js entry)
// Test 10: format=esm with __esModule (should be respected for browser target)
itBundled("cjs/__toESM_format_esm", {
files: {
"/entry.js": /* js */ `
@@ -227,7 +231,7 @@ describe("bundler", () => {
},
format: "esm",
run: {
// __esModule respected because entry is .js (not Node ESM)
// __esModule respected for browser target
stdout: '"the default"',
},
});
@@ -247,17 +251,17 @@ describe("bundler", () => {
},
format: "cjs",
run: {
// __esModule respected because entry is .js (not Node ESM)
// __esModule respected for browser target
stdout: '"the default"',
},
});
// ============================================================================
// Tests for .mjs files re-exporting from .cjs
// .mjs files ARE in Node ESM mode, so __esModule is IGNORED
// Tests for .mjs files re-exporting from .cjs (browser target)
// For browser target, __esModule is ALWAYS respected, even for .mjs files
// ============================================================================
// Test 12: .mjs re-exporting default from CJS
// Test 12: .mjs re-exporting default from CJS (browser target)
itBundled("cjs/__toESM_mjs_reexport", {
files: {
"/entry.js": /* js */ `
@@ -273,12 +277,12 @@ describe("bundler", () => {
`,
},
run: {
// .mjs wraps entire module as default
// Browser target: entire module wrapped as default (no __esModule)
stdout: '{"foo":"foo","bar":"bar"}',
},
});
// Test 13: .mjs re-exporting with __esModule (IGNORED in .mjs)
// Test 13: .mjs re-exporting with __esModule (browser target - RESPECTED)
itBundled("cjs/__toESM_mjs_reexport_with_esModule", {
files: {
"/entry.js": /* js */ `
@@ -295,12 +299,12 @@ describe("bundler", () => {
`,
},
run: {
// __esModule IGNORED in .mjs - entire module wrapped as default
stdout: '{"__esModule":true,"default":{"value":"from cjs"},"other":"other"}',
// Browser target: __esModule IS RESPECTED, default gets exports.default
stdout: '{"value":"from cjs"}',
},
});
// Test 14: Deep re-export chain through .mjs files
// Test 14: Deep re-export chain through .mjs files (browser target)
itBundled("cjs/__toESM_deep_reexport_chain", {
files: {
"/entry.js": /* js */ `
@@ -322,7 +326,7 @@ describe("bundler", () => {
},
});
// Test 15: Re-export with rename from .mjs
// Test 15: Re-export with rename from .mjs (browser target)
itBundled("cjs/__toESM_reexport_with_rename", {
files: {
"/entry.js": /* js */ `
@@ -471,7 +475,7 @@ describe("bundler", () => {
`,
},
run: {
// __esModule respected in .js entry, default gets the default property
// __esModule respected, default gets the default property
stdout: '{"value":"nested"}',
},
});
@@ -539,10 +543,10 @@ describe("bundler", () => {
});
// ============================================================================
// Tests for the original issue #26901 - .js file importing CJS with __esModule
// Tests for the original issue #26901 - browser target scenarios
// ============================================================================
// Test 24: Original issue case - .js importing from CJS with __esModule
// Test 24: .js importing from CJS with __esModule (browser target)
itBundled("cjs/__toESM_issue_26901_js_file", {
files: {
"/entry.js": /* js */ `
@@ -555,12 +559,12 @@ describe("bundler", () => {
`,
},
run: {
// .js file: __esModule is RESPECTED, so default import gets exports.default
// Browser target: __esModule is RESPECTED
stdout: '{"msg":"hello"}',
},
});
// Test 25: Same case but with .mjs entry - __esModule is IGNORED
// Test 25: .mjs importing from CJS with __esModule (browser target)
itBundled("cjs/__toESM_issue_26901_mjs_file", {
files: {
"/entry.mjs": /* js */ `
@@ -573,13 +577,13 @@ describe("bundler", () => {
`,
},
run: {
// .mjs file: __esModule is IGNORED, so default import gets entire module
stdout: '{"__esModule":true,"default":{"msg":"hello"}}',
// Browser target: __esModule is RESPECTED even for .mjs entry
stdout: '{"msg":"hello"}',
},
});
// Test 26: package.json "type": "module" - __esModule is IGNORED
itBundled("cjs/__toESM_issue_26901_type_module", {
// Test 26: "type": "module" with browser target - __esModule is RESPECTED
itBundled("cjs/__toESM_issue_26901_type_module_browser", {
files: {
"/entry.js": /* js */ `
import lib from './lib.cjs';
@@ -593,10 +597,75 @@ describe("bundler", () => {
exports.default = { msg: 'hello' };
`,
},
// Default target is browser
run: {
// "type": "module" makes .js files Node ESM mode
// __esModule is IGNORED, so default import gets entire module
// Browser target: __esModule is RESPECTED even with "type": "module"
stdout: '{"msg":"hello"}',
},
});
// ============================================================================
// Tests for Node.js target with Node ESM files
// Only this combination ignores __esModule
// ============================================================================
// Test 27: target=node with .mjs entry - __esModule is IGNORED
itBundled("cjs/__toESM_node_target_mjs_entry", {
files: {
"/entry.mjs": /* js */ `
import lib from './lib.cjs';
console.log(JSON.stringify(lib));
`,
"/lib.cjs": /* js */ `
exports.__esModule = true;
exports.default = { msg: 'hello' };
`,
},
target: "node",
run: {
// Node.js target + .mjs entry: __esModule is IGNORED (Node.js behavior)
stdout: '{"__esModule":true,"default":{"msg":"hello"}}',
},
});
// Test 28: target=node with "type": "module" - __esModule is IGNORED
itBundled("cjs/__toESM_node_target_type_module", {
files: {
"/entry.js": /* js */ `
import lib from './lib.cjs';
console.log(JSON.stringify(lib));
`,
"/package.json": /* json */ `
{ "type": "module" }
`,
"/lib.cjs": /* js */ `
exports.__esModule = true;
exports.default = { msg: 'hello' };
`,
},
target: "node",
run: {
// Node.js target + "type": "module": __esModule is IGNORED (Node.js behavior)
stdout: '{"__esModule":true,"default":{"msg":"hello"}}',
},
});
// Test 29: target=node with regular .js entry - __esModule is RESPECTED
itBundled("cjs/__toESM_node_target_js_entry", {
files: {
"/entry.js": /* js */ `
import lib from './lib.cjs';
console.log(JSON.stringify(lib));
`,
"/lib.cjs": /* js */ `
exports.__esModule = true;
exports.default = { msg: 'hello' };
`,
},
target: "node",
run: {
// Node.js target + .js entry (NOT Node ESM): __esModule is RESPECTED
stdout: '{"msg":"hello"}',
},
});
});