如何在给定索引下将一个不变列表中的所有元素插入到另一个中

Question

I need to insert the contents of an array (pure JS) into an immutable List at a given index.

I have a list like [a,b,c,d,e] which represents the ids at indexes 0 - 4 on the server. I then fetch an array [j,k,l,m,n] (which represents the id's at indices 9 - 14 ). I want to merge the two lists so that I have:

[a,b,c,d,e,undefined,undefined,undefined,undefined,undefined,j,k,l,m,n]

It's possible to do oldList.insert(9, nextList) but this nests the whole nextList array into index 9 and not it's contents at indices 9 - 14

I need something like the ES6/Next spread operator..

const ids = oldList.insert(10, ...next_ids)

But this isn't possible.

Any ideas?

Answer 1

Here is my current solution but be interested to know if there is a more succinct way..

const list = List( ['a', 'b', 'c', 'd', 'e'] ) // Indices 0 - 5
const fetchResponse = ['j', 'k', 'l', 'm', 'n'] // Indices 9 - 14
const nextList = List()
  .setSize(9) // The start index of fetched results
  .concat(List(fetchResponse))
  .mergeWith((next, last) => next || last, list) // Take new value at index if defined, otherwise old

console.log(nextList.toJS())
  -> ['a', 'b', 'c', 'd', 'e', null, null, null, null, null, 'j', 'k', 'l', 'm', 'n']