FirstLastMiddleElementsOfArray_5.0.ts

// JavaScript Version

export const buildFirstMiddleLastList = (list) => {
  const { 0: first, [list.length - 1]: last, ...rest } = list;

  return {
    first: first,
    middle: Object.values(rest),
    last: list.length > 1 ? last : undefined,
  };
};

const arrayData = ['a', 'b', 'c', 'd', 'e'];
const {first, middle, last} = buildFirstMiddleLastList(arrayData);

first; // "a"
middle; // ["b", "c", "d"]
last; // "e"

FirstLastMiddleElementsOfArray_5.1.ts

// Jest Test

describe('buildFirstMiddleLastList()', () => {
  test.each`
    input                                   | expected
    ${[]}                                   | ${{ first: undefined, middle: [], last: undefined }}
    ${[1]}                                  | ${{ first: 1, middle: [], last: undefined }}
    ${[1, 2]}                               | ${{ first: 1, middle: [], last: 2 }}
    ${[1, 2, 3, 4, 5]}                      | ${{ first: 1, middle: [2, 3, 4], last: 5 }}
    ${['a', 'b', 'c', 'd', 'e']}            | ${{ first: 'a', middle: ['b', 'c', 'd'], last: 'e' }}
    ${[{ a: 'a' }, { b: 'b' }, { c: 'c' }]} | ${{ first: { a: 'a' }, middle: [{ b: 'b' }], last: { c: 'c' } }}
  `('returns $expected when buildFirstMiddleLastList($input)', ({ expected, input }) => {
    expect(buildFirstMiddleLastList(input)).toEqual(expected);
  });
});

FirstLastMiddleElementsOfArray_5.2.ts

// TypeScript Version

type UnArray<T> = T extends Array<infer U> ? U : T;

interface IFirstMiddleLastList<T> {
  first: UnArray<T> | undefined;
  last: UnArray<T> | undefined;
  middle: T;
}

export const buildFirstMiddleLastList = <T extends any[]>(list: T): IFirstMiddleLastList<T> => {
  const { 0: first, [list.length - 1]: last, ...rest } = list;

  return {
    first: first,
    middle: Object.values(rest) as T,
    last: list.length > 1 ? last : undefined,
  };
};