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This is quite likely to be in the training data, since it's one of the projects in Wes Bos's free 30 days of Javascript course[0].

I was under the impression for this to work like that, training data needs to be plenty. One project is not enough since it’s too "sparse".

But maybe this example was used by many other people and so it proliferated?

The repo[0] currently has been forked ~41300 times.

It’s quite unlikely that training data will include duplicate repositories or even forks, that alone would surpass the published dataset sizes.