> How is it invalid? Here's the syntax about a for..in loop

   for (variable in object) {
     statement
   }

And here's the syntax for Vue's for..in loop. Only one of it is somewhat Javascript:

   item in items
   (item, index) in items
   (val, key) in object
   (val, name, index) in object

And, of course, there's an extension to that

   v-for="item in items" :key="item.id"

Edit: additionally, from that very link: "for...in should not be used to iterate over an Array where the index order is important." But this doesn't concern Vue, it maintains the order in its for..in.