Rendering a List
The key prop
Now we are trying to look at the key prop. See:
But when rendering arrays, React needs a way to identify each element with a unique key. It doesn't have to be a number. But it should be a unique value.
If you don't, React will scream like this:
Warning: Each child in a list should have a unique "key" prop. Check the render method of `Page`. See https://fb.me/react-warning-keys for more information. in li (created by Page) in Page