Learning about recursion using Javascript

recursion.js

// this function takes an array and double each component inside it
const double = (nums) => {
  if(nums.length === 0) {
    return []
  } else {
    return [2 * nums[0]].concat(double(nums.slice(1, nums.length)))
  }
}

console.log(double([1,2,3,4]), "should be [ 2, 4, 6, 8 ]")

// this function remove odd element inside the array
const removeOdd = (nums) => {
  if(nums.length === 0) {
    return []
  } else {
    if(nums[0] % 2 === 0) {
      return [nums[0]].concat(removeOdd(nums.slice(1, nums.length)))
    } else {
      return removeOdd(nums.slice(1, nums.length))
    } 
  }
}

console.log(removeOdd([1,2,3,4,5,6,7,7,8,9]), "should be [ 2, 4, 6, 8 ]")