How to set environment variables from within package.json?

How to set some environment variables from within package.json to be used with npm start like commands?

Here's what I currently have in my package.json:

{
  ...
  "scripts": {
    "help": "tagove help",
    "start": "tagove start"
  }
  ...
}

I want to set environment variables (like NODE_ENV) in the start script while still being able to start the app with just one command, npm start.

Set the environment variable in the script command:

...
"scripts": {
  "start": "node app.js",
  "test": "NODE_ENV=test mocha --reporter spec"
},
...

Then use process.env.NODE_ENV in your app.

Note: This is for Mac & Linux only. For Windows refer to the comments.