状況
Next.js を使用したアプリケーションのテストで、transformには swc/jest を利用している。
テストコードの中で、 testing-library の render() で
react-markdown を内部で利用しているコンポーネントをrenderすると、エラーが出てしまう。
Jest encountered an unexpected token
Jest failed to parse a file. This happens e.g. when your code or its dependencies use non-standard JavaScript syntax, or when Jest is not configured to support such syntax.
Out of the box Jest supports Babel, which will be used to transform your files into valid JS based on your Babel configuration.
By default "node_modules" folder is ignored by transformers.
Here's what you can do:
• If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/ecmascript-modules for how to enable it.
• If you are trying to use TypeScript, see https://jestjs.io/docs/getting-started#using-typescript
• To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
• If you need a custom transformation specify a "transform" option in your config.
• If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.
You'll find more details and examples of these config options in the docs:
https://jestjs.io/docs/configuration
For information about custom transformations, see:
https://jestjs.io/docs/code-transformation
SyntaxError: Unexpected token 'export'
これは react-markdown がESMであることに由来しているようだ。
環境
"@swc/core": "1.4.5",
"@swc/jest": "0.2.36",
"@testing-library/jest-dom": "6.4.2",
"@testing-library/react": "14.2.2",
"jest": "29.7.0",
"typescript": "5.2.2"
対処
https://github.com/vercel/next.js/issues/40183#issuecomment-1979587339
こちらの方法を参考に、 jest.config.js を書き換えることで回避できた 
jest.config.js
const nextJest = require('next/jest')
const createJestConfig = nextJest({
dir: './',
})
const esModules = [
'react-markdown',
'swiper',
// その他ESMのものを記述
].join('|')
const customJestConfig = {
cacheDirectory: 'node_modules/.cache/jest',
moduleNameMapper: {
'^~/(.*)$': '<rootDir>/src/$1',
'^@/(.*)$': '<rootDir>/$1',
},
testEnvironment: 'jest-environment-jsdom',
transform: {
'^.+\\.(t|j)sx?$': [
'@swc/jest',
{
jsc: {
transform: {
react: {
runtime: 'automatic',
},
},
},
},
],
},
extensionsToTreatAsEsm: ['.ts', '.tsx'],
setupFilesAfterEnv: ['./jest.setup.js'],
// 〜以下略〜
}
module.exports = async () => ({
...(await createJestConfig(customJestConfig)()),
transformIgnorePatterns: [`<rootDir>/node_modules/(?!${esModules})/`],
})
transformIgnorePatterns を createJestConfig()の外側で後から上書きするのがポイントらしい。
https://github.com/vercel/next.js/issues/36077#issuecomment-1096635363