array-manipulations.js

const replace = (arr, i, newEl) => arr.map((el, k) => {
  return k === i ? newEl : el
})

const replaceWhere = (arr, pred, transform) => arr.map((el, i) => {
  return pred(el, i) ? transform(el, i) : el
})

const swap = (arr, i, j) => arr.map((el, k) => {
  if (k === i) return arr[j]
  if (k === j) return arr[i]
  return el
})

// Usage:

const reducer = state => {
  ...state,
  todos: swap(state.todos, i, j)
}