Node build command not working

I am using this config in package.json file for running webpack dev server. I want to use a build command as well, but its not working for me

{
  "name": "Demo",
  "version": "1.0.0",
  "description": "Demo App",
  "main": "index.js",
  "scripts": {
    "start": "webpack-dev-server --hot",
    "build": "webpack --watch"
},

npm start command works fine, but build isn't

via h_a86