FirstLastMiddleElementsOfArray_4.0.ts

// JavaScript Version

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

  return [
    first, 
    Object.values(rest), 
    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_4.1.ts

// Jest Test

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

FirstLastMiddleElementsOfArray_4.2.ts

// TypeScript Version

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

type FirstMiddleLastListReturn<T> = [UnArray<T> | undefined, T, UnArray<T> | undefined];

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

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