Global object

In Node every function and variable should be scoped to a module. We should not define functions and variables within the global scope.

In Node the equivalent to the browser's Window object is global. The properties and methods that belong to this method are available anywhere in a program.
Just as we can technically write Window.console.log(), we can write global.console.log() however in both cases it is more sane to use the shorthand.
However if we declare a variable in this scope in browser-based JavaScript, this variable becomes accessible via the Window object and thus is accessible in global scope. The same is not true for Node. If you declare a variable at this level it will return undefined.
This is because of Node's modular nature. If you were to define a function foo in a module and then also define it in the global scope, when you call foo, the Node interpreter would not know which function to call. Hence it chooses not to recognise the global foo, returning undefined.