Production deployments will vary in many ways, but a standard convention when deploying in production is to define an environment variable called NODE_ENV and set its value to “production”.

Runtime flags

Any code running in your application (including external modules) can check the value of NODE_ENV:

if(process.env.NODE_ENV === 'production') {
    // We are running in production mode
} else {
   // We are running in development mode
}

Dependencies

When the NODE_ENV environment variable is set to ‘production’ all devDependencies in your package.json file will be completely ignored when running npm install. You can also enforce this with a --production flag:

npm install --production

For setting NODE_ENV you can use any of these methods

method 1: set NODE_ENV for all node apps

Windows :

set NODE_ENV=production

Linux or other unix based system :

export NODE_ENV=production

This sets NODE_ENV for current bash session thus any apps started after this statement will have NODE_ENV set to production.

method 2: set NODE_ENV for current app

NODE_ENV=production node app.js

This will set NODE_ENV for the current app only. This helps when we want to test our apps on different environments.

method 3: create .env file and use it

This uses the idea explained here. Refer this post for more detailed explanation.

Basically you create .env file and run some bash script to set them on environment.