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

Easy way to make immutable object in JS is to use Object.freeze.

I created your input array as:

const array1 = Object.freeze([
    Object.freeze([ 1, 2 ]),
    Object.freeze([ 3, 4 ])
])

Then, when I tried to call your function I got:

Uncaught TypeError: Cannot add/remove sealed array elements
at Array.shift ()
at arr.reduce (:7:38)
at Array.reduce ()
at roundRobin (:4:28)
at :6:17

You're using shift, which is mutating orignal array.

Replace it with Array.slice(0,1)[0] and it will start working.