I thought .map was for arrays??

This post provides context on why Results have a map method.

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Yes, you're right. If you were to ask a JS developer what .map does, they'd probably give you their own definition of the Array.prototype.map method.

But what if I told you that .map is an implementation of a more abstract idea?

I will not get into the details (because they are completely unnecessary for the purposes of this discussion in all honesty), but one thing I do want to tell you about is that in the world of functional programming there's this thing called a Functor.

What's a Functor? A Functor is a "container" that can be mapped over.

So, what's an example of a Functor? ...... JS Array's are Functors! They are containers that can be mapped over.

[1, 2, 3].map(num => `number is: ${num}`)

map takes a Array and turns it into a Array with a callback function as an argument. Or more formally:

Array<T>.map<U>(fn: (val: T) => U): Array<U>

So what else is a container that you use every day that could be a Functor? JS Maps, plain objeccts, Sets, and basically anything that is a "container" for data.

Suppose I had a Map<string, number>. Then I could map over the contents of the Map (the number) just like I would with an array - the string keys would be untouched.

So, with all that out of the way, I think we can agree that a Result is a container for some data. And .map for Results is conceptually the exact same thing as Array.prototype.map.