TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" after upgrade to 9.0.3

Prerequisites

• Checked that your issue hasn’t already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn’t just a feature that actually isn’t supported in the environment in question or a bug in your code.
• ‘Smoke tested’ the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

I just tried upgrading to 9.0.3 and i’m getting a TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts". Running (obviously) on typescript, with the command TS_NODE_PROJECT='./tsconfig.test.json' mocha -r ts-node/register -r tsconfig-paths/register 'src/**/*.test.ts'. This is my tsconfig.test.json:

{
  "compilerOptions": {
    "moduleResolution": "node",
    "target": "es5",
    "module": "commonjs",
    "lib": ["es2015", "es2016", "es2017", "esnext", "dom"],
    "strict": true,
    "sourceMap": true,
    "declaration": false,
    "strictNullChecks": false,
    "allowSyntheticDefaultImports": true,
    "noImplicitAny": false,
    "resolveJsonModule": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "esModuleInterop": true,
    "baseUrl": "src",
    "paths": {
      "@pages/*": ["pages/*"],
      "@components/*": ["components/*"],
    },
    "typeRoots": ["node_modules/@types"]
  },
  "include": ["src"]
}

This used to work in 9.0.1

Steps to Reproduce

Run above code with the configuration on a .ts file using ES modules.

Expected behavior: The file is being loaded and executed

Actual behavior: An error is thrown TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts".

Reproduces how often: 100%

Versions

• The output of mocha --version and node node_modules/.bin/mocha --version: 9.0.2
• The output of node --version: v14.16.1
• Your operating system
  • name and version: MacOS
  • architecture (32 or 64-bit): 64bit
• Your shell (e.g., bash, zsh, PowerShell, cmd): zsh

[buesing]

Yeah, I’m sticking with version 1.1.0 of the slugify package for now. Thanks!

[juergba]

@buesing Yes, with this topic you are stumbling into an experimental minefield. You will have to spend considerable amount of time to get into it, or to get out again. I can give you some tipps, then I will kindly lead you to the exit.

• esm package: hasn’t been updated for over two years (=abandonded) and is not supported in Mocha v9. Mocha supports Node’s native ESM support.
• sindresorhus guide: to read
• I recommend option 3) and pin @sindresorhus/slugify: ^1.1.0 (version before pur ESM)
• otherwise --loader=ts-node/esm: see this guide. --loader is an experimental Node option (alias of --experimental-loader) and ts-node/esm is experimental loader hook.
• sindresorhus guide: Can I import ESM packages in my TypeScript project? => yes, but you need to convert your project to output ESM. Your target is ES5. Why?
• sindresorhus guide:
  • Use only full relative file paths for imports
  • You must use a .js extension in relative imports even though you're importing .ts files

Edit: or don’t update to Mocha v9, stay on v8.