Mastering JavaScript Functional Programming - Second Edition

Functions as objects

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

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

This is (almost) equivalent to writing the following:

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

However, this is not true for hoisting. In this case, JavaScript moves all definitions to the top of the current scope, but does not move the 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 ...