JavaScript: Functional Programming for JavaScript Developers

Functors

While morphisms are mappings between types, functors are mappings between categories. They can be thought of as functions that lift values out of a container, morph them, and then put them into a new container. The first input is a morphism for the type and the second input is the container.

Note

The type signature for functors looks like this:

// myFunctor :: (a -> b) -> f a -> f b

This says, "give me a function that takes a and returns b and a box that contains a(s), and I'll return a box that contains b(s).

Creating functors

It turns out we already have one functor: map(). It grabs the values within the container, an array, and applies a function to it.

[1, 4, 9].map(Math.sqrt); // Returns: [1, 2, 3]

However, we'll need to write it as a global ...