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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
I think arr.reduce is more clear than an arr.forEach that increments an external value - arr.reduce((sum, val) => sum + val, 0)
The problem stated that the array would have at least one value; no need to check if (len === 0) for this problem.
Also, it's not clear, but it might be that any array of less than 3 values might need to return -1. (The tests seem to only use arrays of length 3 or more.)