Question

var main = {
   doSomething: function(){
      function firstOne(){}
      function secondOne(){
         $.ajax({
            success: function(){
               firstOne(); /* here I want the previous function */
            }
         });
      }
   }
}

How can I run the firstOne() function from the marked line? It’s a noobish question perhaps, but I don’t get the JS namespace (and I tried).

Is that kind of defining functions good from a JS good practices point of view?

Answer 1

What you do works. This passage of MDN explains why:

You can nest a function within a function. The nested (inner) function is private to its containing (outer) function. It also forms a closure.

…

Since a nested function is a closure, this means that a nested function can "inherit" the arguments and variables of its containing function. In other words, the inner function contains the scope of the outer function.

To summarize:

The inner function can be accessed only from statements in the outer function.

The inner function forms a closure: the inner function can use the arguments and variables of the outer function, while the outer function cannot use the arguments and variables of the inner function.

So this explains why you can call firstOne from within the Ajax callback.

Whether this is good design or not depends completely on what you are trying to achieve. If firstOne is something that you could reuse in several parts of your code, then it would certainly be a bad decision to define it as a nested function, where it can only be accessed in a local context. But if you had for example two Ajax calls within secondOne that both needed the same functionality in their callback, then it would be a good decision to wrap this in a local nested function. If you just need the behaviour once then it might be overkill (and extra typing) to declare it as a separate function.