function hello(str){
        alert(str);
}

{
  "name": "webpackdemo",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo "Error: no test specified" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "webpack": "^4.1.1"
  }
}

webpack hello.js hello.bundle.js

ERROR in multi ./hello.js hello.bundle.js
Module not found: Error: Can't resolve 'hello.bundle.js' in 'D:webpackDemo'
 @ multi ./hello.js hello.bundle.js