#1: create package.json
npm init

#2: install node-sass
npm install node-sass --save-dev

Insert the below code for running "node-sass" to the "script" value.
"start": "node-sass -w --output-style expanded scss/style.scss css/style.css"

{
  ...,
  "scripts": {
    ...,
    "start": "node-sass -w --output-style expanded scss/style.scss css/style.css"
  },
  ...
}

node-sass [options] <input> [output]

quote source (node-sass usage)

npm run start

Run to code for "node-sass" by javascript

node compile.js