Well there’s no reliable way to detect running in Node.js since every website could easily declare the same variables, yet, since there’s no window object in Node.js by default you can go the other way around and check whether you’re running inside a Browser.
This is what I use for libs that should work both in a Browser and under Node.js:
if (typeof window === 'undefined') {
exports.foo = {};
} else {
window.foo = {};
}
It might still explode in case that window is defined in Node.js but there’s no good reason for someone do this, since you would explicitly need to leave out var or set the property on the global object.
EDIT
For detecting whether your script has been required as a CommonJS module, that’s again not easy. Only thing commonJS specifies is that A: The modules will be included via a call to the function require and B: The modules exports things via properties on the exports object. Now how that is implement is left to the underlying system. Node.js wraps the module’s content in an anonymous function:
function (exports, require, module, __filename, __dirname) {
See: https://github.com/ry/node/blob/master/src/node.js#L325
But don’t try to detect that via some crazy arguments.callee.toString() stuff, instead just use my example code above which checks for the Browser. Node.js is a way cleaner environment so it’s unlikely that window will be declared there.