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【Jest】JestはCJSにしか対応しておらず、ESMで書くとそのままでは対応できない

要旨

JestによるESMのサポートはExperimental(実験的)のため、適用には手順が必要。

詳細

CJS (CommonJS)とESM (ECMAScript Modules) の違いをよく理解しないまま「ESMの書き方の方がかっこいいやん」と思ってESMで書き始めたところ、jestでテストしようとすると下記のエラーに出くわした。

/app # npm test

> fake-kabus-websocket-server@1.0.0 test
> jest

 FAIL  ./time_delta.test.js
  ● Test suite failed to run

    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

    Details:

    /app/time_delta.test.js:3
    import { TimeDelta } from "./time_delta.js"; // const TimeDelta = require('./time_delta.js');
    ^^^^^^

    SyntaxError: Cannot use import statement outside a module

      at Runtime.createScriptFromCode (node_modules/jest-runtime/build/index.js:1796:14)

ESMの 実験的な サポートはあるにはあるようだが、変なところで詰まるのも嫌なので素直にCJSで書き直そうと思う。

jestjs.io

参考

jestjs.io