what's up with this ^{:key... map starting the line? https://github.com/day8/re-frame/blob/master/examples/todomvc/src/todomvc/views.cljs#L67

Tells reagent (and react) to add (:id todo) as the key of the todo-item

It’s merging {:key (:id todo)} into the metadata on the todo-item component

which reagent uses to fill in the key property

you can also do it inside the todo-item component, by putting {:key (:id todo)} in the top level attributes

this is a good explanation of what a key is used for: https://reactjs.org/docs/lists-and-keys.html