[joncrocks]

I believe it's best practice to set the key to be something that is based on the identity of the component to be rendered.

The idea is that this is used to suggest whether when rendering a component, the children are new components, or simply existing components with updated props.

E.g. You have a component that renders a list of items. If you add an item, what you would rather do is create a single new child component and re-use the previously generated children vs. discarding the child components from a the previous render and creating a brand new set.

If you create a key based on a hash of the props this will decrease the time taken to re-render an set of child components with identical props, but at the cost of discarding an existing component and creating a new one if one of the props changes.

Or something like that.....

See for a bit more detail: https://facebook.github.io/react/docs/reconciliation.html