TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts"

The error "TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension '.ts'" occurs when Node.js encounters a TypeScript (.ts) file but doesn't recognize it natively, as Node.js only supports JavaScript (.js) files by default. To run TypeScript files in Node.js, you need to transpile them to JavaScript first.

To resolve this issue, you can use a build tool like tsc (TypeScript Compiler) or a bundler like Webpack or Babel to transpile your TypeScript files into JavaScript before executing them with Node.js.

Here's a step-by-step guide to resolve the error:

1. Install TypeScript: Make sure you have TypeScript installed in your project. If it's not installed, you can install it globally using npm:

   bashCopy code
   npm install -g typescript
   

2. Compile TypeScript to JavaScript: Before running your Node.js application, you need to compile your TypeScript files to JavaScript using the tsc command. If you haven't already, create a tsconfig.json file in the root of your project to specify TypeScript compilation options.

   jsonCopy code
   // tsconfig.json
   {
     "compilerOptions": {
       "target": "es6",
       "outDir": "./dist",
       "rootDir": "./src",
       "esModuleInterop": true
     }
   }
   

   In this example, we are setting the outDir to ./dist, which means the compiled JavaScript files will be placed in the dist directory. You may adjust this as per your project structure.

3. Compile the TypeScript Files: Open your terminal, navigate to your project directory, and run the TypeScript compiler (tsc):

   bashCopy code
   tsc
   

   This will transpile your TypeScript files into JavaScript files and place them in the specified outDir (e.g., ./dist) as per your tsconfig.json.

4. Run the Transpiled JavaScript Files: After successfully compiling your TypeScript files, you can run the transpiled JavaScript files using Node.js. For example:

   bashCopy code
   node ./dist/index.js
   

   Replace index.js with the entry file of your Node.js application.

By following these steps, your TypeScript code will be transpiled into JavaScript, allowing Node.js to run it without encountering the "ERR_UNKNOWN_FILE_EXTENSION" error.