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[Rich-Harris]

including

• you can return an object or a promise
• it runs on client and server
• it will try and serialize data to avoid running it on both sides
• you can use this.redirect and this.error
• only top-level components can use it
• it's optional
• you can access req on the server, which is good for session-based stuff (maybe get into authentication? or maybe that should be left for examples)