How to fix nodemon not found in npm with JavaScript?

To fix nodemon not found in npm with JavaScript, we install nodemon.

To install it, we run

npm install nodemon --save 

Then we add

{
  //...
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "start": "nodemon app.js"
  }
  //...
}

in package.json to add the start script to run app.js with nodemon.

Then we run

npm start

to start the script.