Install

npm install --save nodejs-detector

Usage:

var isNode = require('nodejs-detector');

if (isNode) {
  console.log("Running under Node.JS");
} else {
  alert("Hello from browser (or whatever not-a-node env)");
}

The check is performed as:

module.exports = false;

// Only Node.JS has a process variable that is of [[Class]] process
try {
 module.exports = Object.prototype.toString.call(global.process) === '[object process]' 
} catch(e) {}

This check is both the most reliable I could find and it does not use process env directly, which would cause browserify to include it into the build.