Generate a linked list of people with support for couples, used to create the christmas chain (one person gives only to one other person).

unique-linked-list-with-couples.js

// Unique list of people, list in list is a couple that for some reason only can give to a specific person.
const list = [
  ['person1', 'person2'], // couple
  'person3',
  'person4',
  ['person5', 'person6'], // couple
  'person7',
];

// randomize the list of who gives to who.
const shuffle = arr => {
  const array = [...arr];
  const copy = [];
  let n = arr.length;
  let i;

  // While there remain elements to shuffle.
  while (n) {
      // Pick a remaining element.
    n -= 1;
    i = Math.floor(Math.random() * n);

    // And move it to the new array.
    copy.push(array.splice(i, 1)[0]);
  }

  return copy;
};

// flattens the array into a one dimensional list.
const flatten = arr => [].concat(...arr.map( v => v instanceof Array ? flatten(val) : [val]))

// print the list of who gives to who.
const print = arr => {
  const flat = flatten(arr);
  for (let i = 0; i < flat.length; i += 1) {
    console.log(flat[i], '=>', flat[(i + 1) % flat.length]);
  }
};

print(shuffle(list));

A version that does not need the condition in the loop.

// unpacks the array into a one dimensional list.
const concat = arr => arr.reduce((acc, val) => acc.concat(val instanceof Array ? val : [val]), []);

// print the list of who gives to who.
const print = arr => {
  const unpacked = concat(arr);
  for (let i = 0; i < unpacked.length; i += 1) {
    console.log(unpacked[i], '=>', unpacked[(i + 1) % unpacked.length]);
  }
};

The inner arrays (the person who need to give to a specific person) are unpacked to a single linear list.

list = [
  [person1, person2],
  person3,
]

becomes:

list = [person1, person2, person3]

With this structure the print method will be a lot simpler.

Tested with the following seedable js random, to check the same result is achieved as in previous version.

// https://stackoverflow.com/questions/521295/seeding-the-random-number-generator-in-javascript
let seed = 1;
function random() {
  const x = Math.sin(seed++) * 10000;
  return x - Math.floor(x);
}

Nice work! Added it now.

Do you mean that it would be faster with the random number generator or just that you tested it with that?