calculate a number of all possible combinations from an array of numbers

function combinations(arr) {
  let array = []
  for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr.length; j++) {
      array.push(arr[i] + arr[j]);
    }
  }
  return array;
}

combinations([1, 2, 3, 4, 5])