Node ExpressJS - events.js: 167 throw er Unhandled error event

I have installed mocha chai in working expressjs application using the following commands:

npm install mocha chai sinon supertest --save-dev

When I run the application with: npm run start, I have the following errors:

events.js:167
  throw er; // Unhandled 'error' event
  ^
Error: listen EADDRINUSE :::8001
at Server.setupListenHandle [as _listen2] (net.js:1334:14)
at listenInCluster (net.js:1382:12)
at Server.listen (net.js:1469:7)
at Object.<anonymous> (folderName/app.js:33:24)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)

How to fix this?