shell
npm install --save detect-node

js
var isNode = require('detect-node');

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

js
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) {}