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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.
As far as I understand, length is a property of the array that remains constant. So it is a simple property access to do "this.length". It is not a method, nothing is being calculated.
edit: But thank you for at least explaning the logic behind the usage!
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It's not necessary to say "len = this.length". The more common way to write for loops is simply:
Per the description, startDate and endDate are already Date objects.