The Javascript Shuffle

Since there is no guarantee that the two random numbers will be different, it’s possible that a card switches with itself and some cards will never be switched.

Let’s improve this code by forcing the swap on each card in the deck and eliminating the need to generate two random numbers:function altNaiveShuffle(deck) { let randomCard; let tempX; for (let index = deck.

length – 1; index > -1 ; index -= 1) { randomCard = Math.


random() * deck.

length); tempX = deck[index]; deck[index] = deck[randomCard]; deck[randomCard] = tempX; }}This alternative is better, with less code, but does not guarantee that the position of randomCard will not equal the index.

We can make a small alteration which will match an algorithm that fixes this problem.

Fisher-Yates ShuffleA single line change in the function above will morph it into implementing the Fisher-Yates algorithm.

This change will randomly choose cards that have not been iterated through, avoiding the possibility that a card switches with itself.

randomCard = Math.


random() * index);The random pool of cards to choose from will be one less than the current index due to the mechanism of Math.


In other words, it’s targeting the set of remaining cards that are eligible for a swap.

Value vs.

 PositionThe code above works great.

It’s performant and optimal.

But there is one aspect that bothers me.

Instead of moving the position of each card, it’s swapping the values.

In a real-world recreation, it would be equivalent to this:1.

Remember the value of the card in position A2.

Draw over the card in position A with a value from position B3.

Draw over the card in position B with the remembered value4.

Repeat this process for the duration of the shuffleIt seems a bit dirty to me.

Shuffling by changing the face value of a card is like moving the labels of a Rubik’s Cube to solve it.

Let’s use a bit of code from an earlier section to shuffle the cards by position instead of value, using Array.

splice() but adhering to the Fisher-Yates algorithm:function spliceShuffle(deck) { let count = deck.

length; let tempX; while(count) { tempX = deck.



random() * count), 1); deck.

splice(count, 0, tempX[0]); count -= 1; }}This solution feels cleaner.

But wait, now there are two splice operations.

Could this be improved?.Yes, let’s try a different mechanism.

Stack ShuffleI’m always looking to improve my coding, including refactoring, to reduce the size or optimize performance.

The shuffle function can be reduced further, by using native prototypical functions built into the Array model, like Array.

push() and a little experimentation:function stackShuffle(deck) { let count = deck.

length; while(count) { deck.




random() * count), 1)[0]); count -= 1; }}The stack push moves the card to the bottom of the array (by order) which walls it from the random selection, keeping it in line with the Fisher-Yates algorithm.

Riffle ShuffleThe stack shuffle represents one of the most optimal implementations (in code-size) of the Fisher-Yates algorithm, but it does not follow how shuffling is done in the real world.

It is a simulation where the result presents itself as a shuffled deck of cards.

It does not mimic the mechanics of a riffle shuffle, the most widely used technique for shuffling a deck of cards.

Here is how a riffle shuffle works in the physical world:1.

Split the deck approximately in half2.

One half in the left hand, the other half in the right hand3.

Riffle the edges of both sets so they intermingle4.

Push the cards together5.

Repeat the process for 6 more timesHow can we alter the stackShuffle function to represent a physical riffle shuffle?.Assuming that a cut deck will not always be exactly 26 cards, we will introduce a random variant of -4 to +4 on one half.

When the cards are riffled, they will not intermesh exactly one card apart; therefore, another variant of 1–3 cards should be introduced.

Then both sides are meshed back together from the bottom up.

There is also a 50 percent chance that the left side will start before the right side.

In reality, I assume the dealer’s dominant hand would lead.

Let’s codify all of those variants into the shuffle function:function riffleShuffle(deck) { const cutDeckVariant = deck.

length / 2 + Math.


random() * 9) – 4; const leftHalf = deck.

splice(0, cutDeckVariant); let leftCount = leftHalf.

length; let rightCount = deck.

length – Math.


random() * 4); while(leftCount > 0) { const takeAmount = Math.


random() * 4); deck.

splice(rightCount, 0, .


splice(leftCount, takeAmount)); leftCount -= takeAmount; rightCount = rightCount – Math.


random() * 4) + takeAmount; } deck.

splice(rightCount, 0, .

leftHalf);}As shown, the riffleShuffle function is relatively complicated compared with the stackShuffle.

It requires multiple shuffles due to the possibility that the first or last card on either side of the deck would remain the same, depending on how the deck is cut and which side starts the riffle.

Shuffling seven times is the suggested number of shuffles to produce a substantially randomized deck when using the riffle technique.

Sometimes experimenting with alternate solutions will amplify a previous one and promote that as the clear winner.

For any of my future card-related projects, I would stick with the stackShuffle as it is the most compact version.

Less code equates to lower defect counts.

Iterate and ImproveThere are probably numerous ways to code a shuffle function.

As long as it fits your needs and creates the effect of a shuffled set of items, then continue to use it.

But challenge yourself to think of alternative strategies and methods to improve your coding skills and reduce lines of code while optimizing for performance.

Revisiting past projects may be helpful to start with an existing code-base.

Refactor code by iterating through small changes and leveraging the full capabilities of a programming language.

A continuous approach to improve your code by iterating small changes will eventually yield an optimal solution.


. More details

Leave a Reply