Self-defining functions.

main.js

    // Self-defining functions
    var greetings = function () {

        console.log("Hello!");

        greetings = function () {

            console.log("Goodbye!");

        }

    };
    greetings(); // Hello!
    greetings(); // Goodbye!

    // Note that if you assign 'greetings' to another variable,
    // that second variable will always point to the last 'greetings' state.
    var goodbye = greetings;
    goodbye(); // Goodbye!
    goodbye(); // Goodbye!