NOTE: This question had two parts, but because the title was “Environment detection: node.js or browser” – I will get to this part first, because I guess many people are coming here to look for an answer to that. A separate question might be in order.
In JavaScript variables can be redefined by the inner scopes, thus assuming that environment has not created variables named as process, global or window could easily fail, for example if one is using node.js jsdom module, the API usage example has
var window = doc.defaultView;
After which detecting the environment based on the existence of window variable would systematically fail by any module running under that scope. With the same logic any browser based code could easily overwrite global or process, because they are not reserved variables in that environment.
Fortunately there is a way of requiring the global scope and testing what it is – if you create a new function using a new Function() constructor, the execution scope of this is bound to the global scope and you can compare the global scope directly to the expected value. *)
So to create a function check if the global scope is “window” would be
var isBrowser=new Function("try {return this===window;}catch(e){ return false;}");
// tests if global scope is bound to window
if(isBrowser()) console.log("running under browser");
And function to test if global scope is bound to “global” would be
var isNode=new Function("try {return this===global;}catch(e){return false;}");
// tests if global scope is bound to "global"
if(isNode()) console.log("running under node.js");
the try… catch -part will makes sure that if variable is not defined, false is returned.
The isNode()could also compare this.process.title==="node" or some other global scope variable found inside node.js if you will, but comparing to the global should be enough in practice.
http://jsfiddle.net/p6yedbqk/
NOTE: detecting the running environment is not recommended. However, it can be useful in a specific environment, like development and testing environment which has some known characteristics for the global scope.
Now – the second part of the answer. after the environment detection has been done, you can select which environment based strategy you want to use (if any) to bind your variable which are “global” to your application.
The recommended strategy here, in my opinion, would be to use a singleton pattern to bind your settings inside a class. There is a good list of alternatives already in SO
Simplest/cleanest way to implement a singleton in JavaScript
So, it may turn out if you do not need a “global” variable, and you do not need the environment detection at all, just use the singleton pattern to defined a module, which will store the values for you. OK, one can argue that the module itself is a global variable, which in JavaScript it actually is, but at least in theory it looks a bit cleaner way of doing it.
*) https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function
Note: Functions created with the Function constructor do not create
closures to their creation contexts; they always are created in the
global scope. When running them, they will only be able to access
their own local variables and global ones, not the ones from the scope
in which the Function constructor was called.