At the end of Chapter 17, Jem understands that Atticus has been able to suggest that Bob Ewell is the one who had beaten Mayella. Scout realizes this as well, but she thinks that Tom Robinson is still a viable suspect. (She does not yet know that Tom's left hand is useless.) Both Jem and Scout are hopeful at this point, but Scout is more skeptical. She ends the chapter, saying, "I thought Jem was counting his chickens."
At the beginning of Chapter 21, Atticus has finished his final statement. Calpurnia walks into the court to notify Atticus that his children are missing. Mr. Underwood informs him that they are in the balcony. He summons them downstairs. Jem approaches and says "We've won, haven't we?" Atticus allows them to return to the court after dinner. Jem is more than hopeful; he is confident. He supposes that they have won and asks if the jury will acquit Tom that quickly. When the kids return to court, Jem expresses his confidence to Reverend Sykes:
Jem smiled. “He’s not supposed to lean, Reverend, but don’t fret, we’ve won it,” he said wisely. “Don’t see how any jury could convict on what we heard—”
Jem became even more emotionally involved in Tom's case than Scout. When the jury returns with a guilty verdict, Jem can not believe it. He had been full of hope. The beginning of Chapter 22 illustrates his frustration about the verdict.