How is joy presented in A Christmas Carol?

Joy is presented primarily as sociality in A Christmas Carol. It is a generous outpouring of oneself and one's resources for the benefit of other people. For example, a key scene that illustrates joy is the Fezziwig Christmas dance.
The Ghost of Christmas Past takes Scrooge to see a Christmas party his old employer, Fezziwig, once threw for his employees and friends. Fezziwig has prepared a feast of food and drink for his young employees, followed by dancing and merriment. Fezziwg spends his money freely so that the people around him can enjoy themselves fully. Watching this scene from his youth brings back a flood of memories so joyful, and yet painful in terms of how Scrooge now lives, that he can hardly stand it.
Likewise, at the end of the book, Scrooge expresses his overwhelming joy at still being alive and able to make amends for his faults by reaching out to other people in generous gestures. He has a giant turkey sent to the Cratchits, gives to charity, and gladly attends his nephew's Christmas party. In his joy, he no longer wants to hoard his money but wants to use it for the general good.