Items of array are unique in order

unique-in-order.test.ts

import { uniqueInOrder } from './unique-in-order'

const obj1 = { id: '1' }
const obj2 = { id: '2' }
const obj3 = { id: '3' }

const arr1 = [1, 2, 3]
const arr2 = [4, 5, 6]
const arr3 = [7, 8, 9]

test('`uniqueInOrder` should work correctly', () => {
  expect(uniqueInOrder([])).toBe(true)

  expect(uniqueInOrder([1])).toBe(true)
  expect(uniqueInOrder(['a'])).toBe(true)
  expect(uniqueInOrder([obj1])).toBe(true)
  expect(uniqueInOrder([arr1])).toBe(true)

  expect(uniqueInOrder([1, 2, 3])).toBe(true)
  expect(uniqueInOrder(['a', 'b', 'c'])).toBe(true)
  expect(uniqueInOrder([obj1, obj2, obj3])).toBe(true)
  expect(uniqueInOrder([arr1, arr2, arr3])).toBe(true)

  expect(uniqueInOrder([1, 2, 2, 3])).toBe(false)
  expect(uniqueInOrder(['a', 'b', 'b', 'c'])).toBe(false)
  expect(uniqueInOrder([obj1, obj2, obj2, obj3])).toBe(false)
  expect(uniqueInOrder([arr1, arr2, arr2, arr3])).toBe(false)
})

unique-in-order.ts

/**
 * Checks that there are no consecutive repeated elements in the array, for example:
 * ['a', 'b', 'c', 'd'] -> TRUE
 * ['b', 'b', 'C', 'C', 'd'] -> FALSE
 */
export const uniqueInOrder = <T = unknown>(arr: T[]): boolean => {
  return arr.every((item, index) => item !== arr[index + 1])
}