Server side rendering with async data fetch. Something like a static fetchData() method is the correct way to handle data fetching with React Router in the general case, though it can reach down into child components as needed (which is e.g. how Relay works).
The reason you want to do it this way is that React Router resolves all the matched routes all at once. Given that, you can then run data fetching for all of your route handlers simultaneously.
If instead you tied data fetching to instance-level handlers on components, you’d always end up with fetch waterfalls, where a component could not fetch its required data until all of its parents receive their required data, and so forth. While that may not be a big problem on the server, it’s hugely suboptimal on the client.
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