Under what doctrine was Rahab saved at Jericho?

When the Israelites attacked Jericho, God commanded them not spare the residents of Jericho, or Canaanites generally. Two spies went into the city, and were protected by a harlot named Rahab, who asked them to spare her life and those of her family in return. The spies were able to honor the deal and she became an Israelite. What made her the exception to the "kill all" command? Was it her faith that saved her?