[Feature]: Support loading of ".cjs" and ".cts" files.

[smack0007]

🚀 Feature Proposal

Support loading and transforming of files with the ".cts" file extension.

Motivation

In our code base we import all our files using the ".js" file extension even though we are writing in TypeScript. We then use a custom jest resolver to load files when running the tests in jest as it will not recognize the files otherwise. We also have situations where we differentiate between CommonJS and ESM implementations using ".cts" and ".mts" files extensions which means the imports then have the extensions ".cjs" and ".mjs" respectively. Using ".cjs" imports in a test leads to the following warning:

ts-jest[ts-jest-transformer] (WARN) Got a unknown file type to compile (file: ./shared/src/file.cts). To fix this, in your Jest config change the `transform` key which value is `ts-jest` so that it does not match this kind of files anymore.

And the file is not transformed at all and so the test fails. In our custom resolver we have inserted the hack displayed in the example section which allows our ".cts" files to loaded without any problems.

Example

// HACK: ts-jest does not consider .cts files as TypeScript
/** @type { { TS_TSX_REGEX: RegExp, JS_JSX_REGEX: RegExp }  } */
const constants = require('ts-jest/dist/constants');
constants.TS_TSX_REGEX = /\.[cm]?tsx?$/;
constants.JS_JSX_REGEX = /\.[cm]?jsx?$/;

[ahnpnl]

What you are doing in your codebase is a valid solution. The thing is that ts-jest will transform files which are given by Jest. To be able to load those file extensions, it’s in Jest scope because it’s related to module resolution and module loader, but not transformer area.

You should request this to Jest team about loading files.

about transforming more file extensions, you will need to adjust your transform config to have the regex to include the extensions you need. This belongs to Jest config and ts-jest cannot interfere with that process.

[michael42]

I don't think I get what you're saying, here's a reproduction repo, maybe that helps.

There, you can easily see, that jest supports .cjs just fine. One can import .cjs and use that within a test:

x.cjs:

exports.x = ".cjs";

x.spec.js:

const {x} = require('./x.cjs');

it('should work', () => {
    expect(x).toBe('.cjs');
});

But when doing the same, but with TypeScript:
y.cts:

export const y = '.cts' as const

y.spec.js:

const {y} = require('./y.cts');

it('should work', () => {
    expect(y).toBe('.cts');
});

If ts-jest is properly listed in jest.config.js (transform: {"\.cts$": "ts-jest" }), ts-jest complains that one should remove it there:

ts-jest[ts-jest-transformer] (WARN) Got a unknown file type to compile (file: .../ts-jest-cts/ts/y.cts). To fix this, in your Jest config change the `transform` key which value is `ts-jest` so that it does not match this kind of files anymore.

Which is confusing instead of helpful, because when not processing TypeScript, jest obviously does not support the syntax and throws the usual Jest encountered an unexpected token error.

edit: forgot the repo link

[ahnpnl]

You should check this to understand more about Jest infra https://jestjs.io/docs/code-transformation

[michael42]

What are you suggesting exactly? When jest is configured to call ts-jest, to transform the source (.cts) to JavaScript, but ts-jest only prints a warning and does not transform the code, how does reading docs help?

[ahnpnl]

Ah I know what it is now, because of this

ts-jest/src/constants.ts

Line 4 in abdbaec

export const TS_TSX_REGEX = /\.m?tsx?$/

The issue is about: transforming file, not loading file :) PR is welcome