When using inline functions/callbacks like in this example, they have access to the parent function scope along with all its variables and arguments, known as "lexical scope".
If the callback was not declared inline, it wouldn't have access to those variables/arguments because it would be in a different scope.
Therefore, the inline callbacks for readFile() can access cb(...), and similarly, the inline callback for getUsers() can access req and res since they are part of the parent scope.
It's important to note that this access is not from the call stack but rather from the lexical scope.
The ability to use res.render() only exists because the function is declared within the lexical scope where res is accessible. If the route were declared differently:
function getUsersCallback(err, users) {
if(err){
res.render('error', {error:err})
} else{
res.render('index', {title: "users", users: users.users});
}
}
app.get('/', (req,res) => {
getUsers(getUsersCallback);
});
In this case, it would not work because the callback function isn't declared in a lexical scope that has access to req and res.
Remember that function arguments and local variables in JavaScript are stored in scope objects, not on the call stack like in languages such as C/C++. This allows for function scopes to potentially exist beyond when the function returns (due to closures) and enables scopes to be chained for the interpreter to look up values in the parent scope to implement the "lexical scope" of the language.