getLength

getLength.js

// Write a fn `getLength` that takes one argument, an array, and returns the length of that array. 
// getLength should be written without using the built-in .length() method, and without any `for` or `while` loops.

const getLength = ([first, ...rest], len = 0) => 
  !first
  ? len
  : getLength(rest, len + 1)


// ES5
function getLength2 (arr) {
  if (arr[0] === undefined) return 0
  else {
    var rest = arr.slice(1)
    return 1 + getLength2(rest)
  }
}