On day 58: https://www.hackingwithswift.com/books/ios-swiftui/filtering-fetchrequest-using-nspredicate I dont know why below code works?

          let ship1 = Ship(context: moc)
          ship1.name = "Enterprise"
          ship1.universe = "Star Trek"

When you "let ship1", why you can still change the name and universe properties? I mean ... why not "var ship1"

Can someone explain for me?

It's because Ship is a class, not a struct.

With classes, let means that you cannot change the instance that the variable points to—so you can't replace one Ship with another Ship—but you can change the properties of the instance that the variable points to.

let ship1 = Ship(context: moc)
ship1.name = "Enterprise"
ship1.universe = "Star Trek"

//this will not work
ship1 = Ship(context: moc)
ship1.name = "Millennium Falcon"
ship1.universe = "Star Wars"

//this will work
ship1.name = "Defiant"

thanks! I got it ^_^