How to map over key-values in a map to produce another map with ImmutableJS?

Question

How, using ImmutableJS, do I produce a new map by mapping over the key/value pairs of an input map?

In Scala, I would do something like this:

scala> Map(1->2, 3->4).toSeq.map{case (k, v) => (k*k) -> (v*v*v)}.toMap
res1: scala.collection.immutable.Map[Int,Int] = Map(1 -> 8, 9 -> 64)

(This case trivially squares the key and cubes the value.)

In JavaScript, I'm hoping for something like:

Immutable.fromJS({1: 2, 3: 4}).map((v, k) => [k*k, v*v*v]).toJS()
// { 1: 8, 9: 64 }

(I'm using ES6 via Babel.)

I realize that this isn't guaranteed to produce a defined result, due to the potential key collision. In my application I am preventing this elsewhere.

I'm actually using an OrderedMap, if that makes a difference.

Cratch

There is a map() function available facebook.github.io/immutable-js/docs/#/Map/map – Hinckley

Hi Ben, the function passed to map is expected to return a new value. I'm looking for something that's expected to produce a new key/value pair, because I need to change both. – Cratch

Good point, I clearly didn't the question properly – Hinckley

Answer 1

The function I was looking for is mapEntries. Here's an example:

Immutable.fromJS({1: 2, 3: 4}).mapEntries(([k, v]) => [k*k, v*v*v]).toJS()
// { 1: 8, 9: 64 }

mapEntries takes a function that takes a key/value pair as an array, and should return the new key/value pair also as another array.

Cratch

Answer 2

Alternatively:

Immutable.fromJS({ 1: 2, 3: 4 })
  .entrySeq()
  .map(([k, v]) => [k*k, v*v*v])

• discussion: https://github.com/immutable-js/immutable-js/issues/667
• docs: https://immutable-js.com/docs/v4.3.7/Map/#entrySeq()
• playground: https://neilff.github.io/immutable-repl/?q=ZnJvbUpTKHsxOiAyLCAzOiA0fSkKICAuZW50cnlTZXEoKQogIC5tYXAoKFtrLCB2XSkgPT4gW2sqaywgdip2KnZdKQo%3D

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