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How to Develop an Actual Online Slot Machine Game from Scratch (With Code Template)

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Developing your own slot machine game can be an enjoyable programming project as well as good practice for honing your coding skills. Maybe you’ve tried an impressive slot machine from Betsoft or any other popular game developer, and want to try creating your own.

In this comprehensive guide, I’ll break down exactly how to create a fully functional online slot machine game from the ground up using HTML, CSS and JavaScript.

Slot Machine Game Design

Before diving into development, you’ll want to sketch out the basics for your slot machine game on paper:

  • Theme – Space, jungle, mystical, steampunk, etc. This impacts symbols and graphics
  • Reels & Symbols – Classic slot machines have 3 reels spinning 3 symbols on each reel. Define symbols like bars, 7s, stars, etc.
  • Paylines – Paths winning combinations follow. Often based on middle row or diagonal.
  • Bet Controls – Set basic wagers with + and – buttons
  • Spin Button – Trigger randomizing reels algorithm on-click
  • Win Logic – Code reel positions to detect symbol matches on enabled paylines
  • Credit Meter – Displays current bankroll balance updates

Flesh out other secondary features like music, denominations, meters tracking stats, special bonus and free spin modes as desired.

HTML Structure

HTML will provide the basic content containers and static page elements surrounding the animated JavaScript bits.

<!DOCTYPE html> 



    <title>Lucky 777 Slot!</title>



  <div id=”game-container”>

    <div id=”reels”></div>

    <div id=”controls”>

      <p>Bet: <span id=”bet”>1</span></p> 

      <button id=”spin”>Spin</button>


    <p>Credits:<span id=”credits”>100</span></p>


  <script src=”slotmachine.js”></script>




We have <div> elements to envelop the spinning reels, bet controls that update inner HTML, a Spin button to trigger the game, and credits to track the player’s bankroll.

CSS Styling

With HTML structure in place, add graphic styling like backgrounds, animations and positioning through CSS:

#game-container {

  text-align: center;

  padding: 20px; 


#reels {

  border: 1px solid black;

  width: 250px; 

  margin: 0 auto;


#reels img {

  width: 70px;

  border: 1px solid #ccc;

  box-shadow: 2px 2px 5px #333;  


#spin { 

  padding: 10px 15px;

  background: #8e44ad;

  border: none; 

  border-radius: 5px;

  color: white; 

  font-size: 18px;



This adds a purple spin button, reels container with images representing slot symbols, and some polish. Tweak further once functionality is working.

JavaScript Game Logic

Now for the brains – first setting up variables tracking game state that require updating as we play:

// Slot machine symbols 

let symbols = [‘🍇’,’🔔’,’🍊’,’⭐’,’🍋’,’7′];

// Spin results

let spinResult = [];

// Player credits   

let credits = 100;

// Current bet

let bet = 1;

Next we need a spinReels() function that generates random reel outcomes and checks for winners:

function spinReels() {

  // Generate random symbols for each reel

  spinResult[0] = pickRandomSymbol(); 

  spinResult[1] = pickRandomSymbol();

  spinResult[2] = pickRandomSymbol();

  // Display reels (later handle win evaluation here too)



// Helper to pick random symbol

function pickRandomSymbol() {

  let randomIndex = Math.floor(Math.random() * symbols.length);  

  return symbols[randomIndex];


To complete the spinning illusion, we redraw the reels displaying the randomly selected symbols with some slot machine animation flourish using a loop:

function displayReels() {

  // Loop through reels spinning

  for(let i = 0; i < spinResult.length; i++){

    // Create DOM element for symbol  

    let symbol = document.createElement(“img”);

    symbol.src = spinResult[i] + “.png”;

    // Append to reels container  


    // Spin each symbol css animation


    // Wait 0.1 seconds    



    }, 100 * (i+1)); 



Wiring Up User Interaction

So now we have the foundations for a slot machine: randomly generated spins and visual feedback. Next task – make it actually playable!

We add event listeners to respond on clicking bet adjust controls and the spin button:

// Bet click handlers

document.getElementById(“bet-down”).addEventListener(“click”, function(){ 



document.getElementById(“bet-up”).addEventListener(“click”, function(){



// Spin button click handler

document.getElementById(“spin”).addEventListener(“click”, function(){

  // Check we have credits

  if(credits > 0) {





Supporting functions handle betting and overall slot machine round flow:

function adjustBet(amount) {

  bet += amount;

  // Min/max limits

  if(bet < 1) bet = 1; 

  if(bet > 5) bet = 5;

  // Update displays 




function playRound() {

  // Deduct bet amount 

  credits -= bet;


  // Handle spin


  // TODO: Evaluate win state 

  // Reset reels after spin

  document.getElementById(“reels”).innerHTML = “”;



Implementing Slot Payline Wins

The final piece of our slot machine development puzzle involves payline mechanics to detect, evaluate and reward winning results:

// Define paylines

const paylines = [

  [0, 1, 2], // horizontal

  [0, 3, 6]  // diagonal


function checkWins() {

  // Loop active paylines

  for(let line of paylines) {

    let symbols = [];

    // Push reel symbols landing on payline




    // Check 3 matching symbols

    if(symbols[0] == symbols[1] && symbols[1] == symbols[2]){





function triggerWin(symbol) {

  // Lookup symbol value

  let winValue = 0;

  if(symbol == “🍇”) { winValue = 5; }  

  if(symbol == “🔔”) { winValue = 10; }

  // Update player credits   

  credits += winValue * bet;

  // Show win overlay

  displayWin(symbol, winValue);


There you have it! A complete slot machine game you can now dress up with fancy graphics, expand reels and paylines, add enticing features like free spins and bonus rounds, connect a back-end to track metrics…sky’s the limit!

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