How To Solve ‘TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension “.ts” for…’ Error

3 min readSep 9, 2024

If you’re using Chai 5, Mocha, and ts-node together, probably you may have encountered the error specified in the title.

First, let’s reproduce the issue again:

Create an empty project.

$ mkdir sample-project &&\
cd sample-project &&\
npm init -f

Install the project dependencies.

$ npm install - save-dev mocha \
typescript \
ts-node \
chai \
@types/mocha \
@types/chai

Create a simple typescript configuration file (tsconfig.json) as the one below:

{
  "compilerOptions": {
    "target": "ES2017",                                  
    "module": "commonjs",                                
    "moduleResolution": "Node",                     
    "sourceMap": true,                               
    "outDir": "./dist",                              
    "esModuleInterop": true,                         
    "strict": true,                                  
    "skipLibCheck": true                            
  }
}

Create a simple test case like the one below without adding any assertion

import { expect } from 'chai';

describe('Example test case', function () {
  it('should complete successfully', function () {
    const var1 = 1;
  const var2 = 2;
  const total = var1 + var2;
  const expectedValue = 3;
  // expect(total).to.eql(expectedValue);
  })
})

Create a ‘test’ script in the scripts section in the package.json file to execute the created test file

// package.json
{
//...
"scripts": {
    "test": "mocha -r ts-node/register '*.test.ts'"
  },
//...
}

First, let’s run the test to see if it does not fail without calling expect() function from the chai library.

npm run test

> sample-project@1.0.0 test
> mocha -r ts-node/register '*.test.ts'



  Example test case
    ✔ should complete successfully


  1 passing (3ms)

If you saw an output like the one above, let’s try to make it failed by uncommenting the line calls expect() function.

expect(total).to.eql(expectedValue);

Re-run the test

npm run test

> sample-project@1.0.0 test
> mocha -r ts-node/register '*.test.ts'


 Exception during run: TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" for /home/ibrahimgunduz/projects/personal/sample-project/demo.test.ts
    at new NodeError (node:internal/errors:371:5)
    at Object.getFileProtocolModuleFormat [as file:] (node:internal/modules/esm/get_format:87:11)
    at defaultGetFormat (node:internal/modules/esm/get_format:102:38)
    at defaultLoad (node:internal/modules/esm/load:21:14)
    at ESMLoader.load (node:internal/modules/esm/loader:359:26)
    at ESMLoader.moduleProvider (node:internal/modules/esm/loader:280:58)
    at new ModuleJob (node:internal/modules/esm/module_job:66:26)
    at ESMLoader.#createModuleJob (node:internal/modules/esm/loader:297:17)
    at ESMLoader.getModuleJob (node:internal/modules/esm/loader:261:34)
    at async Promise.all (index 0)
    at async ESMLoader.import (node:internal/modules/esm/loader:337:24)
    at async importModuleDynamicallyWrapper (node:internal/vm/module:437:15)
    at async formattedImport (/home/ibrahimgunduz/projects/personal/sample-project/node_modules/mocha/lib/nodejs/esm-utils.js:9:14)
    at async Object.exports.requireOrImport (/home/ibrahimgunduz/projects/personal/sample-project/node_modules/mocha/lib/nodejs/esm-utils.js:42:28)
    at async Object.exports.loadFilesAsync (/home/ibrahimgunduz/projects/personal/sample-project/node_modules/mocha/lib/nodejs/esm-utils.js:100:20)
    at async singleRun (/home/ibrahimgunduz/projects/personal/sample-project/node_modules/mocha/lib/cli/run-helpers.js:162:3)
    at async Object.exports.handler (/home/ibrahimgunduz/projects/personal/sample-project/node_modules/mocha/lib/cli/run.js:375:5) {
  code: 'ERR_UNKNOWN_FILE_EXTENSION'

The issue occurs because Chai is migrated to ESM starting from v5.
https://github.com/chaijs/chai-http/releases/tag/5.0.0

Solution 1:

Modify the following options from the tsconfig.json file

{
  // "module": "commonjs",                                
  // "moduleResolution": "Node",
  // "target": "ES2017",
  "module": "nodenext",
  "moduleResolution": "nodenext",
  "target": "esnext",           
}

Create or update your .mocharc.json file in the following shape

// .mocharc.json
{
  //..
    "loader": "ts-node/esm",
    "extensions": ["ts"]
  //..
}%

Run the test again:

npm run test

> sample-project@1.0.0 test
> mocha -r ts-node/register '*.test.ts'

(node:899490) ExperimentalWarning: --experimental-loader is an experimental feature. This feature could change at any time
(Use `node --trace-warnings ...` to show where the warning was created)


  Example test case
    ✔ should complete successfully


  1 passing (5ms)

Solution 2:

Downgrade the Chai package to the latest 4.x version.

You can find the complete source code with the first solution in the following Github repository.

https://github.com/ibrahimgunduz34/blog-code-examples/tree/master/unknown-file-extension-ts

How To Solve ‘TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension “.ts” for…’ Error

Solution 1:

Solution 2:

Credits:

Written by İbrahim Gündüz