recursion.txt

RECURSION = function expressed in terms of itself

function sum(arr, n) {
 if (n <= 0) {
  return 0;
} else {
  return sum(arr, n - 1) + arr[n - 1];
}
}


--return countdown/countup to array--
----push to count up, unshift to count down

function countup(n) {
  if (n < 1) {
    return [];
  } else {
    const countArray = countup(n - 1);
    countArray.push(n);
    return countArray;
  }
}
console.log(countup(5)); // [1, 2, 3, 4, 5]


function countdown(n) {
  if (n < 1) {
    return [];
  } else {
    const countArray = countdown(n - 1);
    countArray.unshift(n);
    return countArray;
  }
}
console.log(countdown(10)); // [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
console.log(countdown(-1)); // []

--generate range of numbers---
function rangeOfNumbers(startNum, endNum) {
  if (endNum - startNum === 0) {
    return [startNum];
  } else {
    var numbers = rangeOfNumbers(startNum, endNum - 1);
    numbers.push(endNum);
    return numbers;
  }
}
ALT1
function rangeOfNumbers(startNum, endNum) {
  return startNum === endNum
    ? [startNum]
    : rangeOfNumbers(startNum, endNum - 1).concat(endNum);
}
ALT2
function rangeOfNumbers(startNum, endNum) {
  return startNum === endNum
    ? [startNum]
    : [...rangeOfNumbers(startNum, endNum - 1), endNum ];
}