Add new object between each object in an array

I want to add a new object between each object in an array using functional programming. The new object is derived using its two neighbors.

Let’s say that we start with an array of integers [1,2,3], I want to add new integers by adding its neighbors so the resulting array looks like this [1,3,2,5,3]

I know of intersperse but that only allows me to add a static element. I also considered chain but that only visits one element at a time

Solution

You can use R.mapAccum to create an array that uses the previous item (saved in the accumulator). Take the result of R.mapAccum, and use R.last to get the array (dropping the accumulator), flatten, and remove the redundant first item:

const { pipe, mapAccum, last, flatten, tail } = R

const fn = pipe(
  mapAccum((a, b) => [b, [a + b, b]], 0),
  last, // take the array of pairs (drop the accumulator)
  flatten, // flatten the array of pairs to an array of numbers
  tail // take all items after the redundant first
)

const numbers = [1, 2, 3]

const result = fn(numbers)

console.log(result) // [1,3,2,5,3]

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Vanilla JS - reduce the array to another array. On each iteration, spread the previous accumulator (acc), create an item by adding the current number to the previous number on the accumulator (or 0 if none), and also include the current number. Slice redundant first element off.

const fn = (cb, arr) => arr.reduce((acc, n) => 
  [...acc, cb(n,  acc.at(-1) ?? 0), n]
, []).slice(1)

const numbers = [1, 2, 3]

const add = (a, b) => a + b
const result = fn(add, numbers)

console.log(result) // [1,3,2,5,3]