node package manager
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detect-node

Usage:

var isNode = require('detect-node');
 
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) {}
 

Thanks to Ingvar Stepanyan for the initial idea. 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.