Array to sequence

to-sequence.test.ts

import { toSequence } from './to-sequence'

test('`toSequence` should work correctly', () => {
  expect(toSequence([])).toEqual([])

  expect(toSequence(['a'])).toEqual([['a', undefined]])

  expect(toSequence(['a', 'b'])).toEqual([['a', 'b']])

  expect(toSequence(['a', 'b', 'c'])).toEqual([['a', 'b'], ['b', 'c']])

  expect(toSequence(['a', 'b', 'c', 'd'])).toEqual([['a', 'b'], ['b', 'c'], ['c', 'd']])

  expect(toSequence([{ id: 'a' }, { id: 'b'}, { id: 'c' }])).toEqual([
    [{ id: 'a' }, { id: 'b' }],
    [{ id: 'b' }, { id: 'c' }],
  ])
})

to-sequence.ts

/**
 * Takes an array of elements and returns an array, the elements of which are arrays of two values,
 * where the first value indicates the previous value of the array or the first element of the passed array,
 * and the second value indicates the current value of the loop or the value of the next element.
 *
 * Examples:
 * toSequence(['a', 'b']) // -> [['a', 'b']]
 * toSequence(['a', 'b', 'c]) // -> [['a', 'b'], ['b', 'c']]
 */
export const toSequence = <T = unknown>(items: T[]): [T | undefined, T | undefined][] => {
  return items.reduce<[T, T][]>((acc, point, index, arr) => {
    if (index !== 1) {
      acc.push(index > 0 ? [arr[index - 1], point] : [point, arr[index + 1]])
    }

    return acc
  }, [])
}