Examples usages of recursion

Example 1: Manipulate function argument until some condition is true

The following is a solution to this kata which describes the challenge as:

So, your task is to sum up all the digits in string, possibly multiple times, till you get a one digit result. You should then return it as a string. The input will be always valid.

const str = '1234'
digitSum(str) // returns 1

/* 1 + 2 + 3 + 4 = 10 <= this result doesn't have one digit => 1 + 0 = 1 */

function digitSum(numStr) {
  // Get digits sum (e.g., 10)   
  const digitsSum = numStr.split('').reduce((sum, num) => sum + Number(num), 0).toString()
  
  // If sum.length === 1, return sum
  if (digitsSum.length === 1)  {
    return digitsSum
  }
  
  // Else, call function again
  return digitSum(digitsSum)
}

Example 2

Solution to kata. Need to elaborate on how this works.

function getElement(array, indices) {
  if (indices.length === 1) {
    return array[indices]
  }
  
  return getElement(array[indices[0]], indices. Slice(1))
}