Fisher–Yates shuffle

shuffle.js

// Great explanation https://bost.ocks.org/mike/shuffle/
// This relies on mutation. Could the algorithm be recreated using immutable data?
function shuffle(array) {
  var count = array.length
  while (count) {
    const current = count - 1
    // exchange current card with a random card below the current card
    swapIndices(array, current, Math.floor(Math.random() * current))
    // decrement count
    count = count - 1
  }
  return array;
}

function swapIndices(array, i, j) {
    const t = array[i]
    array[i] = array[j]
    array[j] = t
}