recursion-simple-examples.js

const countDownFromN = (n) => {
  if (n < 0) {
    return
  }

  console.log('up:', n)
  countDownFromN(n - 1)
  // console.log('do:', n) // This is weird but its due to the call stack. If you understand why this happens, you understand recursion.
}

countDownFromN(10)


// First valid approach. Removing the first element of the array each time
// O(n) Time
// O(n) Space
// iterateRecursive = (arr) => {
//   if (arr.length === 0) {
//     return
//   }

//   let arrayCopy = [...arr] // With the spread op really a copy

//   console.log(arrayCopy[0])
//   arrayCopy.shift()
//   iterateRecursive(arrayCopy)
// }

// Another valid approach. Iterates from the back and uses and index to
// O(n) Time
// O(1) Space
iterateRecursive = (arr, index = arr.length - 1) => {
  if (index < 0 || index >= arr.length) {
    return
  }

  console.log(arr[index])
  iterateRecursive(arr, index - 1)
}

let arr = [1, 2, 3, 4]

iterateRecursive(arr, 3)
// 1
// 2
// 3
// 4

// Accumulating/compounding the results of recursive calls
const addAllNumsToN = (n) => {
  if (n === 0) {
    return 0
  }

  let result = n + addAllNumsToN(n - 1)
  return result;
}

console.log(addAllNumsToN(5))

// n + (n - 1)
// 5 + 4 => 9
// 9 + 3 => 12
// 12 + 2 => 14
// 14 + 1 => 15