codility-lesson2-A.js

function OddOcurrences (A) {
  const len = A.length
  A.sort()

  for (let i = 0; i < len; i++) {
    if (A[i] !== A[i + 1] && A[i] !== A[i - 1]) {
      return A[i]
    }
  }

  return -1
}

// Testing

const cases = [
  {
    label: 'Given',
    input: OddOcurrences([9, 3, 9, 3, 9, 7, 9]),
    shouldBe: 7
  },
  {
    label: 'Last position answer',
    input: OddOcurrences([5, 3, 5, 3, 5, 9, 5]),
    shouldBe: 9
  },
  {
    label: 'First position answer',
    input: OddOcurrences([5, 3, 5, 9, 5, 9, 5]),
    shouldBe: 3
  },
  {
    label: 'No answer',
    input: OddOcurrences([5, 3, 3, 9, 5, 9, 5]),
    shouldBe: -1
  }
]

cases.map(test => console.log((test.input === test.shouldBe ? '✅' : '❗') + '  ' + test.label + ' => ' + 'Should be: ' + test.shouldBe + ' received: ' + test.input))