Does the chain function in underscore.js create a monad?

Question

In the chain documentation you find:

Calling chain on a wrapped object will cause all future method calls to return wrapped objects as well

Answer 1

No, not a monad, but a comonad! It turns a function that takes a wrapped object and returns a normal value into a function that both takes and returns a wrapped object. As a Haskell type signature that would be:

(Wrapped a -> b) -> (Wrapped a -> Wrapped b)

The type signature of value is:

Wrapped a -> a

These are precisely what you need for a comonad. The first function is usually called extend and the second extract.

You can think of a comonad as a value with some extra context. And that is of course exactly what chain does.

See this Stackoverflow question for more about comonads.