ERR_PACKAGE_PATH_NOT_EXPORTED：檔案存在，為什麼 Node.js 不能匯入？

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import assert from 'node:assert/strict';
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import { mkdtemp, mkdir, writeFile, access } from 'node:fs/promises';
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import { tmpdir } from 'node:os';
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import { join } from 'node:path';
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import { pathToFileURL } from 'node:url';
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const root = await mkdtemp(join(tmpdir(), 'exports-check-'));
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const pkg = join(root, 'node_modules', 'demo-exports');
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await mkdir(join(pkg, 'dist'), { recursive: true });
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await writeFile(join(pkg, 'package.json'), JSON.stringify({
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  name: 'demo-exports',
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  version: '1.0.0',
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  type: 'module',
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  main: './dist/internal.js',
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  exports: {
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    '.': './dist/index.js',
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    './feature': './dist/feature.js',
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    './missing': './dist/absent.js',
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  },
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}));
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await writeFile(join(pkg, 'dist', 'index.js'), 'export const value = 1;');
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await writeFile(join(pkg, 'dist', 'feature.js'), 'export const value = 2;');
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await writeFile(join(pkg, 'dist', 'internal.js'), 'export const value = 99;');
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await access(join(pkg, 'dist', 'internal.js')); // 實體檔案確實存在
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const consumer = join(root, 'consumer.mjs');
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await writeFile(consumer, `
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  import assert from 'node:assert/strict';
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  import { value as rootValue } from 'demo-exports';
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  import { value as featureValue } from 'demo-exports/feature';
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  assert.equal(rootValue, 1); // exports 優先，沒有走 main 的 99
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  assert.equal(featureValue, 2);
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  for (const specifier of [
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    'demo-exports/dist/internal.js',
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    'demo-exports/dist/feature.js',
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    'demo-exports/package.json',
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  ]) {
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    await assert.rejects(import(specifier), {
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      code: 'ERR_PACKAGE_PATH_NOT_EXPORTED',
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    });
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  }
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  await assert.rejects(import('demo-exports/missing'), {
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    code: 'ERR_MODULE_NOT_FOUND',
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  });
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`);
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await import(pathToFileURL(consumer).href);
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console.log('公開入口、深層路徑、manifest 與缺檔錯誤檢查通過');
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console.log('Fixture:', root);