Reuse code inside if statement. (Is function inside function bad practice?)

Question

I heard from some tutorials that defining functions inside function is bad practice.

In my case I need to reuse some code inside two nested callbacks. For example:

router.get('...', function(req, res) {
  db.on('load', function(){
    function doRequest() {
      // Valid token required to do the request.
    }

    if (!validToken) {
      getValidToken().then(... doRequest() ...)
    }
    else {
      doRequest()
    }
  });
});

You see the function doRequest() is defined inside other functions (callbacks). Someday in the future I may need to do something like this, but inside defined by me function. Is that wrong?

Do I using bad practice in this "code"? Can I make it better?

Answer 1

If doRequest() is defined inside the scope of the callback, it will only exist on that scope. It's called a closure and it's useful in many situations. So if you only use this function inside this scope, it's not a problem, as you won't be duplicating content.

On the other hand, if doRequest() performs generic functions and does not access data from the callback (without being sent as a parameter), it should be defined outside, so that it can later be reused.

In the end, it's really a matter of balancing code maintenability, performance and data access. Here's a tutorial on scopes and closures.