Javascript: Recursively count the sum of all elements in array?

Question

I'm trying to write a recursive function to count the number of items in a array using Javascript.

I can do it in Python:

def count(list):
 if list == []:
  return 0
 return 1 + count(list[1:]) 

How can I do it in ES5 and ES6?

Answer 1

Counting elements is ridicoulous since you can just take the length property. It will be O(1) and do what you expect. As for summing or doing something with the elements:

// recursively. Works only on arrays 
const sumElements = (arr) => {
  if (arr.length === 1) return arr[0];

  const [e, ...rest] = arr;
  return e + sumElements(rest);
}


// recursively and effiecent. Works only on arrays
const sumElements = (arr) => {
  const helper = (index, acc) => index < 0 ? acc helper(index - 1, acc + arr[index]);
  return helper(arr.length-1, 0);
}


// using higher order functions. Works for all collections that has reduce defined
const sumElements = list => list.reduce((acc, e) => acc + e), 0);

// using iterators. Works for all collections that has iterator defined
const sumElements = (list) => {
  let sum = 0;
  for (const e of list) {
    sum += e;
  }
  return sum;
}