secret_santa_classic.js

function secretSanta(values, mutualEnabled = true) {
    var indexes = [];
    indexes.length = values.length;
    indexes = indexes.fill(0)
        .map((v, i) => { return {index: i, order: Math.random()} })
        .sort((a, b) => a.order - b.order)
        .map(item => item.index)
    ;
    var fraglen = left => {
        let count = Math.max(2, Math.ceil(left * Math.random()));
        return mutualEnabled && left - count > 1 ? count : left;
    };
    var result = [];
    while (indexes.length > 0) {
        indexes.splice(0, fraglen(indexes.length)).forEach((index, i, fragment) => {
            result.push({
                source: values[index],
                target: values[fragment[(i + 1) % fragment.length]],
            });
        });
    }
    return result;
}
console.log('secretSanta mutual ON', secretSanta(['Alice', 'Bob', 'Craig', 'Dave', 'Eve']));
console.log('secretSanta mutual OFF', secretSanta(['Alice', 'Bob', 'Craig', 'Dave', 'Eve'], false));

/*
secretSanta mutual ON [
  { source: 'Alice', target: 'Eve' },
  { source: 'Eve', target: 'Craig' },
  { source: 'Craig', target: 'Alice' },
  { source: 'Bob', target: 'Dave' },
  { source: 'Dave', target: 'Bob' }
]
secretSanta mutual OFF [
  { source: 'Bob', target: 'Craig' },
  { source: 'Craig', target: 'Eve' },
  { source: 'Eve', target: 'Alice' },
  { source: 'Alice', target: 'Dave' },
  { source: 'Dave', target: 'Bob' }
*/