player--shuffle-deck.js

class Player extends GameObject {
    // [... lots o' other stuff ...] 

  shuffleDeck() {
    this.game.emitEvent(
      'onDeckShuffled',
      { player: this }
    )
    this.deck = _.shuffle(this.deck)
    if (
      this.isTopCardOfDeckVisible() &&
      this.deck.length > 0
    ) {
      this.deck[0].facedown = false
      this.deck.slice(1).forEach((card) => {
        card.facedown = true
      })
      this.addTopCardOfDeckVisibleMessage()
    }
  }

  // [... more other stuff ...] 
}

player--take-mulligan.js

class Player extends GameObject {
  // [... lots o' other stuff ...] 

  takeMulligan() {
    let size = this.hand.length
    for (let card of this.hand) {
      this.moveCard(card, 'deck')
    }
    this.shuffleDeck()
    this.drawCardsToHand(size - 1)
    this.takenMulligan = true
  }

  // [... more other stuff ...] 
}

underscore--random.js

function random(min, max) {
  if (max == null) {
    max = min;
    min = 0;
  }
  return min + Math.floor(Math.random() * (max - min + 1));
}

underscore--sample.js

function sample(obj, n, guard) {
  if (n == null || guard) {
    if (!isArrayLike(obj)) obj = values(obj)
    return obj[random(obj.length - 1)]
  }
  var sample = toArray(obj)
  var length = getLength(sample)
  n = Math.max(Math.min(n, length), 0)
  var last = length - 1
  for (var index = 0; index < n; index++) {
    var rand = random(index, last)
    var temp = sample[index]
    sample[index] = sample[rand]
    sample[rand] = temp
  }
  return sample.slice(0, n)
}

underscore--shuffle.js

function shuffle(obj) {
  return sample(obj, Infinity)
}