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fix: preserve TypeScript imports for ESM builds #1319

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fix: preserve TypeScript imports for ESM builds #1319

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86a6d11
Preserve TS imports for ESM builds
Jaksenc May 14, 2026
2fdbf35
Address PR feedback
Jaksenc May 18, 2026
7b6fccb
Fix TypeScript ModuleKind link
Jaksenc May 18, 2026
b3476b6
Clarify ESM import condition fixture
Jaksenc May 19, 2026
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35 changes: 27 additions & 8 deletions src/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -258,6 +258,21 @@ function ncc (
}));
}

const tsCompilerOptions = {
module: 'esnext',
target: 'esnext',
...fullTsconfig.compilerOptions,
allowSyntheticDefaultImports: true,
noEmit: false,
outDir: '//'
};

if (esm && emitsNonEsmModule(tsCompilerOptions.module)) {
// ESM builds need TypeScript to preserve imports so webpack can resolve
// dependencies with "import" conditions instead of "require" conditions.
tsCompilerOptions.module = 'esnext';
}

const compiler = webpack({
entry,
cache: cache === false ? undefined : {
Expand Down Expand Up @@ -365,14 +380,7 @@ function ncc (
options: {
transpileOnly,
compiler: eval('__dirname + "/typescript.js"'),
compilerOptions: {
module: 'esnext',
target: 'esnext',
...fullTsconfig.compilerOptions,
allowSyntheticDefaultImports: true,
noEmit: false,
outDir: '//'
}
compilerOptions: tsCompilerOptions
}
}]
},
Expand Down Expand Up @@ -635,6 +643,17 @@ function ncc (
}
}

function emitsNonEsmModule(moduleKind) {
if (typeof moduleKind === 'string') {
return ['commonjs', 'amd', 'umd', 'system'].includes(moduleKind.toLowerCase());
}

// Values match TypeScript's ModuleKind enum:
// CommonJS=1, AMD=2, UMD=3, System=4.
// https://github.com/microsoft/TypeScript/blob/v5.2.2/src/compiler/types.ts#L7255-L7260
return moduleKind >= 1 && moduleKind <= 4;
}

// this could be rewritten with actual FS apis / globs, but this is simpler
function getFlatFiles(mfsData, output, getAssetMeta, tsconfig, curBase = "") {
for (const path of Object.keys(mfsData)) {
Expand Down
3 changes: 3 additions & 0 deletions test/unit/ts-esm-import-condition/input.ts
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@@ -0,0 +1,3 @@
import { value } from 'esm-only-condition-package';

console.log(value);
1 change: 1 addition & 0 deletions test/unit/ts-esm-import-condition/node_modules/esm-only-condition-package/index.d.ts
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1 change: 1 addition & 0 deletions test/unit/ts-esm-import-condition/node_modules/esm-only-condition-package/index.js
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12 changes: 12 additions & 0 deletions test/unit/ts-esm-import-condition/node_modules/esm-only-condition-package/package.json
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3 changes: 3 additions & 0 deletions test/unit/ts-esm-import-condition/opt.json
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@@ -0,0 +1,3 @@
{
"esm": true
}
13 changes: 13 additions & 0 deletions test/unit/ts-esm-import-condition/output-coverage.js
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@@ -0,0 +1,13 @@
/******/ /* webpack/runtime/compat */
/******/
/******/ if (typeof __nccwpck_require__ !== 'undefined') __nccwpck_require__.ab = new URL('.', import.meta.url).pathname.slice(import.meta.url.match(/^file:\/\/\/\w:/) ? 1 : 0, -1) + "/";
/******/
/************************************************************************/
var __webpack_exports__ = {};

;// CONCATENATED MODULE: ./test/unit/ts-esm-import-condition/node_modules/esm-only-condition-package/index.js
const value = 'from-esm-only-package';

;// CONCATENATED MODULE: ./test/unit/ts-esm-import-condition/input.ts

console.log(value);
13 changes: 13 additions & 0 deletions test/unit/ts-esm-import-condition/output.js
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@@ -0,0 +1,13 @@
/******/ /* webpack/runtime/compat */
/******/
/******/ if (typeof __nccwpck_require__ !== 'undefined') __nccwpck_require__.ab = new URL('.', import.meta.url).pathname.slice(import.meta.url.match(/^file:\/\/\/\w:/) ? 1 : 0, -1) + "/";
/******/
/************************************************************************/
var __webpack_exports__ = {};

;// CONCATENATED MODULE: ./test/unit/ts-esm-import-condition/node_modules/esm-only-condition-package/index.js
const value = 'from-esm-only-package';

;// CONCATENATED MODULE: ./test/unit/ts-esm-import-condition/input.ts

console.log(value);
7 changes: 7 additions & 0 deletions test/unit/ts-esm-import-condition/tsconfig.json
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
{
"compilerOptions": {
"module": "commonjs",

@styfle styfle May 19, 2026

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This is super strange behavior since you have module=commonjs in tsconfig.json here but type=module in package.json

Does this code even compile and run using tsc + node?

I dont understand why ncc should support this 🤔

@Jaksenc Jaksenc May 19, 2026

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No you’re so right, the fixture makes the case look a bit weird.

I was trying to test ncc’s ESM output path instaed of saying that this setup should be run directly with tsc + Node. Lemme clean it up by moving the import-only package into node_modules and making esm: true explicit in the fixture options so the test is clearer.

"moduleResolution": "node",
"target": "es2020"
}
}