How does nodejs express framework judge my environment?

  node.js, question
var env = process.env.NODE_ENV || 'development'

I want to ask, how does he judge my environment? Why is development the default? If I put it in the production environment, how do I need to modify it? ?

This is the read environment variable, if you want to put it into the build environment, please set an environment variable.

NODE_ENV=production