RoundRobin functional approach - why does my function have side effects?

Question

Objective

I am trying to create a Round Robin algorithm ( https://en.wikipedia.org/wiki/Round-robin_scheduling ) in a pure functional way.

This function, is

Answer 1

Array#shift is doing the mutating.

var array = [0, 1, 2, 3, 4];
array.shift(); // -> 0
array; // -> [1, 2, 3, 4];

The easiest way around that is to clone the array. Usually that can be done with Array#concat but since your arrays are nested (though simple) you can do this:

const roundRobin = (arr, results) => {
    arr = JSON.parse(JSON.stringify(arr));
    if (arr.length === 0) return results;
    // ...

If you're concerned that the global JSON makes the function impure, you can abstract that out.

const deepClone = (obj) => JSON.parse(JSON.stringify(obj));
roundRobin(deepClone(array), []);