Functions as objects

The concept of first-class objects means that functions can be created, assigned, changed, passed as parameters, or returned as result of yet other functions, in the very same way that you can do with, say, numbers or strings. Let's start with their definition. When you define a function in the usual way:

function xyzzy(...) { ... }

This is (almost) equivalent to writing:

var xyzzy = function(...) { ... }

Except for hoisting. JS moves all definitions to the top of the current scope, but not assignments; so, with the first definition you can invoke xyzzy(...) from any place in your code, but with the second you cannot invoke the function until the assignment has been executed.

See the parallel with the Colossal Cave Adventure Game? Invoking xyzzy(...) anywhere won't always work! And, if you never played that famous interactive fiction game, try it online -- for example, at http://www.web-adventures.org/cgi-bin/webfrotz?s=Adventure or http://www.amc.com/shows/halt-and-catch-fire/colossal-cave-adventure/landing.

The point we want to make, is that a function can be assigned to a variable -- and can also be reassigned, if desired. In a similar vein, we can define functions on the spot, when they are needed. We can even do this without naming them: as with common expressions, if used only once, then you don't need to name it or store it in a variable.