Spread element magically turns functions into 'not functions'

Question

Suppose I have this simple JavaScript function:

function returnArray(){
    return [1, 2, 3];
}

Further suppose that I then say

<         


        

Answer 1

Your logical error has been pointed out in comments and answers, but let me point out a cleaner, simpler, less bug-prone way to write this which is more in line with basic principles of recursion.

function double([head, ...tail]) {
  if (head === undefined) return [];
  return [2*head, ...double(tail)];
}

In other words, there is only one "base case", namely the empty array, which returns an empty array. Everything else is simple recursion.

You could "functionalize" this further with

function map(fn) {
  return function iter([head, ...tail]) {
    return head === undefined ? [] : [fn(head), ...iter(tail)];
  };
}

const double = map(x => 2*x);
console.log(double([1, 2, 3]));