classsed-graphql-mern-apollo | GraphQL library

Your tutorial may be out of date. In older versions of Apollo Client, data was initially set to an empty object. This way, if your code accessed some property on it, it wouldn't blow up. While this was convenient, it also wasn't particularly accurate (there is no data, so why are we providing an object?). Now, data is simply undefined until your operation completes. This is why the latter code is working -- you don't access any properties on data until after loading is false, which means the query is done and data is no longer undefined.

If you want to destructure data when your hook is declared, you can utilize a default value like this: