Solvedts node Compilation is unbelievably slow

Hi, for some reason we have performance problems when using ts-node in our mocha tests. It takes about 500 ms to compile an empty ts file. I added some console.logs to measure the time to run this very line: const output = service.getEmitOutput(fileName) and this line takes 500 ms to run even though the content of the file is an empty string (the code variable on the line 314 is empty string). Actually, all our tests files take too long to process, while the production files take a few milliseconds to compile. So any *.spec.ts takes about 500 ms to compile, a regular *.ts file takes about 20 ms. Do you have any idea what could be the root cause or how should I debug it more?

We use latest mocha, ts-node and typescript, but we tried some old versions too and the problem persists. The tsconfig.json:

  "compileOnSave": false,
  "compilerOptions": {
    "outDir": "dist",
    "rootDir": ".",
    "declaration": true,
    "target": "es2017",
    "lib": ["es2017"],
    "module": "commonjs",
    "moduleResolution": "node"
  "exclude": [
54 Answers

βœ”οΈAccepted Answer

If anyone can give me temporary access to a slow repo, let me know! I can quickly take a look into the issue and happy to sign any NDA, etc you need. If not, can I get a sense of peoples tsconfig.json that's slow - are you relying on files, include, exclude, rootDir or other to compile with TypeScript?

Let's try with reactions:

  • ❀️ files
  • πŸš€ include
  • πŸ‘€ exclude
  • πŸ˜• rootDir
  • πŸŽ‰ Other

Other Answers:

Have you tried --transpile-only mode?

I've run into similar issue today. After ts-node upgrade from 7.0.1 to 8.0.1 my mocha test suite became very slow (32s vs 4s). Adding TS_NODE_TRANSPILE_ONLY=true resolved my issue. Thanks @cspotcode

--transpile-only does speed things up considerably for me. I don't want to lose the type checking though!

For now, please try using the environment flag from - TS_NODE_FILES=true. I'll attempt refactoring to the newer TypeScript watch API and see if it improves performance in the coming weeks.

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