How to Fix the “Unknown file extension .ts” Error in ts-node

The “Unknown file extension .ts” error occurs in ts-node occurs when "type": "module" is set in your package.json file. To fix it, run the TypeScript file with ts-node --esm my-file.ts, or remove "type": "module" from your package.json file.

For example, in a project with this package.json file:

package.jsonCopied!

{
  "name": "cb-js",
  "version": "1.0.0",
  "main": "index.js",
  "type": "module",
  "license": "MIT",
  "devDependencies": {
    "prettier": "^2.8.1"
  }
}

If you have a TypeScript file in your project, e.g., my-file.ts:

my-file.tsCopied!

const num: number = 10;
console.log(num ** 2);

Running the ts-node index.ts command will result in the ERR_UNKNOWN_FILE_EXTENSION TypeError in ts-node:

To fix the “Unknown file extension ‘.ts'” error in ts-node, run ts-node with the --esm option set:

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ts-node --esm my-file.ts

# Or
ts-node-esm my-file.ts

# Or
node --loader ts-node/esm my-file.ts

To avoid using the --esm flag, add the following to your tsconfig.json file:

tsconfig.jsonCopied!

{
  // other settings...
  "ts-node": {
    "esm": true,
    "experimentalSpecifierResolution": "node"
  }
}

After doing this, you’ll be able to run the TypeScript file with only ts-node:

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ts-node my-file.ts

You may also need to add one or more of the following options to your tsconfig.json file:

tsconfig.jsonCopied!

{
  // other settings..
  "compilerOptions": {
    // other compiler options...
    "esModuleInterop": true,
    "module": "ESNext", // "module": "CommonJS" should work too
    "moduleResolution": "Node"
  },
  "include": ["/**/*.ts"],
  "exclude": ["node_modules"]
}

Remove "type": "module" from tsconfig.json file

Alternatively, instead of doing all of the above, you can simply remove the "type": "module" field from your package.json file to fix the ERR_UNKNOWN_FILE_EXTENSION error in ts-node.

tsconfig.jsonCopied!

{
  "name": "cb-js",
  "version": "1.0.0",
  "main": "index.js",
  // removed: "type": "module",
  "license": "MIT",
  "devDependencies": {
    "prettier": "^2.8.1"
  }
}

And you’ll be able to run the TypeScript file with ts-node successfully.

Compile TypeScript files into JavaScript

Another way to avoid ERR_UNKNOWN_FILE_EXTENSION is to stop using ts-node altogether. Instead, you can compile your TypeScript files with tsc and run the JavaScript output with node – the classic way.

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# Compile with tsc
npx tsc --outDir dist my-file.ts

# Run with node
node dist/my-file.ts

Basic TypeScript compilation setup

Here’s a great way to set up your project for easily compiling and debugging TypeScript files.

First, install the TypeScript compiler:

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npm i tsc

Then specify the src dir and out dir in tsconfig.json, and enable source maps for seamless Typescript debugging:

tsconfig.jsonCopied!

{
  "compilerOptions": {
    // ... other options
    "rootDir": "src", // Location of TypeScript files
    "outDir": "dist", // Location of compiled JavaScript files

    "sourceMap": true // Generate sourcemaps
  }
}

Finally, create a start NPM script that automatically runs the tsc and node commands one after the other:

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{
  // ... other options
  "scripts": {
    "start": "tsc && node index.js"
  }
}

Now you can run the script easily with npm start.

And you can debug TypeScript in VSCode too.

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