Map function in D3, confused about multiple functions passed in

This question can finally be settled as Mike Bostock thankfully reacted to GitHub issue #173 Simplify / clarify rescaleX and rescaleY.:

The answer is: it’s good practice in JavaScript. If the object’s methods happen to be bound to the object, it’s unnecessary to specify this, but one generally shouldn’t assume so unless it is documented (and not likely to change in a future version).