When should I store a function into a variable?

I'm learning JavaScript at the moment and I don't quite understand when to write a function into a variable.

For instance, both of the following code blocks do the exact same thing in Node.js:

 var onReq = function(req, res) {
   res.write('Hello');
 };
 
 http.createServer(onReq).listen(3000);

and

function onReq(req, res) {
   res.write('Hello');
 }
 
 http.createServer(onReq).listen(3000);

Which is the best method to do according to best practices, and why?

Usually I'll only use a var funcName = function(){} when I would need to redefine the action(s) for that function later on. For example:

var foo = function(a){ return a * 2; }
var bar = foo(2);

foo = function(a){ return a / 2; }

bar = foo(bar);

Otherwise, for most purposes (assuming it's not a callback or a modifier) declaring a function "classically" is usually acceptable.

I default to the non-variable function onReq(){} version. It's not a concious decision I've made, but thinking about it brings forth these arguments:

• It looks cleaner.
• It is conceptually simpler: it's just a function, while the other is a function and a variable. It's a small thing, but I find it valuable none the less.
• It assures me that onReq will always refer to that function body - one less thing to consider when reading the code. Sort of like marking a variable as final in Java.
• Keeps me from "accidentally" replacing the function, causing unintended side effects elsewhere.

Here is an explaination:

There is a distinction between the function name and the variable the function is assigned to:

• The function name cannot be changed, while the variable the function is assigned to can be reassigned.
• The function name can be used only within the function's body. Attempting to use it outside the function's body results in an error (or undefined if the function name was previously declared via a var statement).

https://developer.mozilla.org/en/JavaScript/Reference/Functions_and_function_scope

I see that nobody mentioned (directly and practically - which seems this question is about) the reason for which I personally find most useful for storing functions in variables. That reason is dealing with a complex logic that requires a lot of choices (e.g. if's) to determine further course of action for which different functions should be called, possibly with different sets of inputs as well. It makes code cleaner when that further course of action is launched in just one place, at the end of our logic.

function fooA (input) {...};
function fooB (input) {...};
let chosenHandler;
let chosenInput;

// Here we establish which function is relevant
if (someCondition) chosenHandler = fooA;
else chosenHandler = fooB;

// Here we establish which parameter should be used
if (someOtherCondition) chosenInput = 'First input';
else chosenInput = 'Second input';

// Call chosen function with chosen parameters - one place, nice and clean
chosenHandler(chosenInput);

If we tried to call functions directly, the code would get much more messy and get the messier the more complex the logic is.

According to John Resig's JavaScript as a First Language article, your first code block is considered as best practice.

I personally have encountered certain issues with calling local functions (functions declared inside other functions) when using non-variable syntax in some versions of IE (most likely IE8 or lower) while variable syntax did work as expected.

Given that functions should not be declated in global namespace, most of functions are local, and therefore it makes sense to use variable syntax for functions always.

I see some different opinions here, but most of them are more based on what you think is better or not, but i don't see the technical reasons on when to use one or another, but for sure there are technical restrictions or advantages in use one declaration formula or another. I'm really a beginner with javascript and I don't feel confident to advise on it, but I will propose a case in which storing a function in a variable is not functional.

In the code below, defining an Angular filter, if I define the function inside a variable, then I can't successfully call it from the filter method. Could anyone explain me the technical reason on this?

See the code (commented is not working):

angular
	.module('loc8rApp')
	.filter('formatDistance', formatDistance);

function formatDistance(){
		return function (distance) {
	    var numDistance, unit;
	    if (distance && _isNumeric(distance)) {
	     if (distance > 1) {
	        numDistance = parseFloat(distance).toFixed(1);
	        unit = 'km';
	      } else {
	        numDistance = parseInt(distance * 1000,10);
	        unit = 'm';
	      }
	      return numDistance + unit;
	    } else {
	      return "?";
	    }
	  };
};

var _isNumeric = function (n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
};

/*
var formatDistance = function () {
  return function (distance) {
    var numDistance, unit;
    if (distance && _isNumeric(distance)) {
      if (distance > 1) {
        numDistance = parseFloat(distance).toFixed(1);
        unit = 'km';
      } else {
        numDistance = parseInt(distance * 1000,10);
        unit = 'm';
      }
      return numDistance + unit;
    } else {
      return "?";
    }
  };
};
*/

Thank you in advance!