title: Node.js: Is there any documentation about the process.env variable
body: <p>I use <code>process.env</code> a little in my program, it seems this variable have nothing to do with my program， without it my app could work well, too.</p>
  
  <p>So how can I fully use the <code>process.env</code>? Is there any document or tutorial about it?</p>