This tutorial will walk you through a process of creation of a tic-tac-toe game
Built with Git Tutor
Before we actually start writing code, I recommend to install editorconfig plugin for your ide/text editor. It will keep code consistent in terms of line-endings style, indentation, newlines
📄 .editorconfig
root = true
[*]
charset = utf-8
indent_style = space
indent_size = 4
end_of_line = lf
trim_trailing_whitespace = true
insert_final_newline = true
Every web-app needs an html entry-point, this ain't exception, so let's add simple html file
📄 index.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Tic Tac Toe</title>
</head>
<body>
</body>
</html>index.js will be a js main file
📄 src/index.js
console.log('Hello world');Now we need to add script to index.html
📄 index.html
<title>Tic Tac Toe</title>
</head>
<body>
-
+ <script src="./src/index.js"></script>
</body>
</html>
Most likely the codebase will grow, so eventually we'll need some module system. This tutorial is not about setting-up a javascript bundler like webpack, so let's just use es6 modules which are already supported by latest Chrome. To make chrome understand import statement, type attribute should be set to module
📄 index.html
<title>Tic Tac Toe</title>
</head>
<body>
- <script src="./src/index.js"></script>
+ <script src="./src/index.js" type="module"></script>
</body>
</html>
Let's define a game state variable
📄 src/index.js
- console.log('Hello world');
+ const GameState = {
+
+ }
We'll need an information about current player to know whether x or o should be placed on a game field.
📄 src/index.js
const GameState = {
-
+ currentPlayer: 0,
}
0 – x should be placed
1 – o
field property will represent a game state.
That's an array of 9 elements (3 columns x 3 rows) with initial value -1. Simple if (fieldValue > 0) check will work to distinguish empty fields from filled.
📄 src/index.js
const GameState = {
currentPlayer: 0,
+ field: Array.from({ length: 9 }).fill(-1),
}
Now we need to implement a function which will switch a current player. Let's do this with XOR operator. (how xor works).
📄 src/index.js
currentPlayer: 0,
field: Array.from({ length: 9 }).fill(-1),
}
+
+ function changeCurrentPlayer(gameState) {
+ gameState.currentPlayer = 1 ^ gameState.currentPlayer;
+ }
To modify field values in plain array we'll need a function to convert row and col indices to an array index
📄 src/index.js
function changeCurrentPlayer(gameState) {
gameState.currentPlayer = 1 ^ gameState.currentPlayer;
}
+
+ function getArrayIndexFromRowAndCol(rowIndex, colIndex) {
+ return rowIndex * 3 + colIndex;
+ }
Now we'll start handling game turn logic. Create a function placeholder
📄 src/index.js
function getArrayIndexFromRowAndCol(rowIndex, colIndex) {
return rowIndex * 3 + colIndex;
}
+
+ function turn(gameState, rowIndex, colIndex) {
+
+ }
Convert row and col indices to plain array index
📄 src/index.js
}
function turn(gameState, rowIndex, colIndex) {
-
+ const index = getArrayIndexFromRowAndCol(rowIndex, colIndex);
}
If game field already contains some value, do nothing
📄 src/index.js
function turn(gameState, rowIndex, colIndex) {
const index = getArrayIndexFromRowAndCol(rowIndex, colIndex);
+ const fieldValue = GameState.field[index];
+
+ if (fieldValue >= 0) {
+ return;
+ }
}
Put player identifier to a field
📄 src/index.js
if (fieldValue >= 0) {
return;
}
+
+ gameState.field[index] = gameState.currentPlayer;
}
and change current player
📄 src/index.js
}
gameState.field[index] = gameState.currentPlayer;
+ changeCurrentPlayer(gameState);
}
The next thing we need to handle is a "win" state. Lets add helper variables which will contain array indices by rows:
📄 src/index.js
field: Array.from({ length: 9 }).fill(-1),
}
+ const Rows = [
+ [0, 1, 2],
+ [3, 4, 5],
+ [6, 7, 8],
+ ];
+
function changeCurrentPlayer(gameState) {
gameState.currentPlayer = 1 ^ gameState.currentPlayer;
}
cols:
📄 src/index.js
[6, 7, 8],
];
+ const Cols = [
+ [0, 3, 6],
+ [1, 4, 7],
+ [6, 7, 8],
+ ];
+
function changeCurrentPlayer(gameState) {
gameState.currentPlayer = 1 ^ gameState.currentPlayer;
}
and diagonals
📄 src/index.js
[6, 7, 8],
];
+ const Diagonals = [
+ [0, 4, 8],
+ [2, 4, 6],
+ ];
+
function changeCurrentPlayer(gameState) {
gameState.currentPlayer = 1 ^ gameState.currentPlayer;
}
Now let's take a look at some examples of a "win" state
1 -1 0
0 1 -1
-1 -1 1
Winner is 1. Sum of diagonal values equals 3
We can assume that we can detect a winner by getting a sum of each row, col and diagonal values and comparing it to a 0 (0 + 0 + 0) or 3 (1 + 1 + 1)
But here's another example
0 -1 1
1 0 -1
-1 -1 0
A sum of 1st and 2nd row = 0
Sum of both diagonals = 0
Sum of 1st and 3d cols = 0
That's not the right way to go... 😞
💡 Easy fix!
Change initial value of field to -3 😎
📄 src/index.js
const GameState = {
currentPlayer: 0,
- field: Array.from({ length: 9 }).fill(-1),
+ field: Array.from({ length: 9 }).fill(-3),
}
const Rows = [
Ok, now we are good. So let's create a simple sum function
📄 src/index.js
gameState.field[index] = gameState.currentPlayer;
changeCurrentPlayer(gameState);
}
+
+ function sum(arr) {
+ return arr.reduce((a, b) => a + b, 0);
+ }
and a helper function which maps field indices to values
📄 src/index.js
function sum(arr) {
return arr.reduce((a, b) => a + b, 0);
}
+
+ function getValues(gameState, indices) {
+ return indices.map(index => gameState.field[index]);
+ }
function getWinner should find if some row, col or diagonal sum is 0 or 3. Let's get values of all rows
📄 src/index.js
function getValues(gameState, indices) {
return indices.map(index => gameState.field[index]);
}
+
+ function getWinner(gameState) {
+ const rows = Rows.map((row) => getValues(gameState, row));
+ }
and do the same for cols and diagonals
📄 src/index.js
function getWinner(gameState) {
const rows = Rows.map((row) => getValues(gameState, row));
+ const cols = Cols.map((col) => getValues(gameState, col));
+ const diagonals = Diagonals.map((col) => getValues(gameState, col));
}
now let's create a single array of all values in field
📄 src/index.js
const rows = Rows.map((row) => getValues(gameState, row));
const cols = Cols.map((col) => getValues(gameState, col));
const diagonals = Diagonals.map((col) => getValues(gameState, col));
+
+ const values = [...rows, ...cols, ...diagonals];
}
and find if some chunk sum equals 0 or 3
📄 src/index.js
const diagonals = Diagonals.map((col) => getValues(gameState, col));
const values = [...rows, ...cols, ...diagonals];
+
+ let winner = -1;
+
+ values.forEach((chunk) => {
+ const chunkSum = sum(chunk);
+
+ if (chunkSum === 0) {
+ winner = 0;
+ return;
+ }
+
+ if (chunkSum === 3) {
+ winner = 1;
+ return;
+ }
+ });
+
+ return winner;
}
Now let's describe a game loop.
We'll create a generator function to query row and col for each next turn from outside world.
If you are not familliar with generator functions – read this medium post
📄 src/index.js
return winner;
}
+
+ function* gameLoop(gameState) {
+
+ }
Generator should execute until getWinner returns anything but -1.
📄 src/index.js
}
function* gameLoop(gameState) {
+ let winner = -1;
+
+ while (winner < 0) {
+ winner = getWinner(gameState);
+ }
}
it should also make a turn befor each getWinner call
📄 src/index.js
let winner = -1;
while (winner < 0) {
+ const [rowIndex, colIndex] = yield;
+ turn(gameState, rowIndex, colIndex);
winner = getWinner(gameState);
}
Now let's test our gameLoop
Create a mock scenario of a game:
📄 src/index.js
winner = getWinner(gameState);
}
}
+
+ const turns = [
+ [1, 1],
+ [0, 1],
+ [0, 0],
+ [1, 2],
+ [2, 2],
+ ];
Create a game generator object
📄 src/index.js
[1, 2],
[2, 2],
];
+
+ const game = gameLoop(GameState);
+ game.next();
Iterate over game turns and pass each turn to generator
📄 src/index.js
const game = gameLoop(GameState);
game.next();
+
+ turns.forEach(turn => game.next(turn));
After execution of this scenario game generator should finish it execution.
This means that leading .next() call should return an object { value: undefined, done: true }
📄 src/index.js
game.next();
turns.forEach(turn => game.next(turn));
+
+ console.log(game.next());
Let's check it with node.js
node src/index.js
{ value: undefined, done: true }Yay, it works!
Now as a core of a game is ready let's start refactor our index.js and split it in several modules
Drop testing code
📄 src/index.js
}
}
- const turns = [
- [1, 1],
- [0, 1],
- [0, 0],
- [1, 2],
- [2, 2],
- ];
-
const game = gameLoop(GameState);
game.next();
-
- turns.forEach(turn => game.next(turn));
-
- console.log(game.next());
Move everything but gameLoop from index.js to game-state.js.
📄 src/game-state.js
const GameState = {
currentPlayer: 0,
field: Array.from({ length: 9 }).fill(-3),
}
const Rows = [
[0, 1, 2],
[3, 4, 5],
[6, 7, 8],
];
const Cols = [
[0, 3, 6],
[1, 4, 7],
[6, 7, 8],
];
const Diagonals = [
[0, 4, 8],
[2, 4, 6],
];
function changeCurrentPlayer(gameState) {
gameState.currentPlayer = 1 ^ gameState.currentPlayer;
}
function getArrayIndexFromRowAndCol(rowIndex, colIndex) {
return rowIndex * 3 + colIndex;
}
function turn(gameState, rowIndex, colIndex) {
const index = getArrayIndexFromRowAndCol(rowIndex, colIndex);
const fieldValue = gameState.field[index];
if (fieldValue >= 0) {
return;
}
gameState.field[index] = gameState.currentPlayer;
changeCurrentPlayer(gameState);
}
function sum(arr) {
return arr.reduce((a, b) => a + b, 0);
}
function getValues(gameState, indices) {
return indices.map(index => gameState.field[index]);
}
function getWinner(gameState) {
const rows = Rows.map((row) => getValues(gameState, row));
const cols = Cols.map((col) => getValues(gameState, col));
const diagonals = Diagonals.map((col) => getValues(gameState, col));
const values = [...rows, ...cols, ...diagonals];
let winner = -1;
values.forEach((chunk) => {
const chunkSum = sum(chunk);
if (chunkSum === 0) {
winner = 0;
return;
}
if (chunkSum === 3) {
winner = 1;
return;
}
});
return winner;
}📄 src/index.js
- const GameState = {
- currentPlayer: 0,
- field: Array.from({ length: 9 }).fill(-3),
- }
-
- const Rows = [
- [0, 1, 2],
- [3, 4, 5],
- [6, 7, 8],
- ];
-
- const Cols = [
- [0, 3, 6],
- [1, 4, 7],
- [6, 7, 8],
- ];
-
- const Diagonals = [
- [0, 4, 8],
- [2, 4, 6],
- ];
-
- function changeCurrentPlayer(gameState) {
- gameState.currentPlayer = 1 ^ gameState.currentPlayer;
- }
-
- function getArrayIndexFromRowAndCol(rowIndex, colIndex) {
- return rowIndex * 3 + colIndex;
- }
-
- function turn(gameState, rowIndex, colIndex) {
- const index = getArrayIndexFromRowAndCol(rowIndex, colIndex);
- const fieldValue = GameState.field[index];
-
- if (fieldValue >= 0) {
- return;
- }
-
- gameState.field[index] = gameState.currentPlayer;
- changeCurrentPlayer(gameState);
- }
-
- function sum(arr) {
- return arr.reduce((a, b) => a + b, 0);
- }
-
- function getValues(gameState, indices) {
- return indices.map(index => gameState.field[index]);
- }
-
- function getWinner(gameState) {
- const rows = Rows.map((row) => getValues(gameState, row));
- const cols = Cols.map((col) => getValues(gameState, col));
- const diagonals = Diagonals.map((col) => getValues(gameState, col));
-
- const values = [...rows, ...cols, ...diagonals];
-
- let winner = -1;
-
- values.forEach((chunk) => {
- const chunkSum = sum(chunk);
-
- if (chunkSum === 0) {
- winner = 0;
- return;
- }
-
- if (chunkSum === 3) {
- winner = 1;
- return;
- }
- });
-
- return winner;
- }
-
function* gameLoop(gameState) {
let winner = -1;
Export everything gameLoop depends on
📄 src/game-state.js
- const GameState = {
+ export const GameState = {
currentPlayer: 0,
field: Array.from({ length: 9 }).fill(-3),
}
return rowIndex * 3 + colIndex;
}
- function turn(gameState, rowIndex, colIndex) {
+ export function turn(gameState, rowIndex, colIndex) {
const index = getArrayIndexFromRowAndCol(rowIndex, colIndex);
const fieldValue = gameState.field[index];
return indices.map(index => gameState.field[index]);
}
- function getWinner(gameState) {
+ export function getWinner(gameState) {
const rows = Rows.map((row) => getValues(gameState, row));
const cols = Cols.map((col) => getValues(gameState, col));
const diagonals = Diagonals.map((col) => getValues(gameState, col));
and import it in index.js
📄 src/index.js
+ import { GameState, getWinner, turn } from './game-state.js';
+
function* gameLoop(gameState) {
let winner = -1;
Add canvas to index.html
📄 index.html
</head>
<body>
<script src="./src/index.js" type="module"></script>
+ <canvas></canvas>
</body>
</html>
and get a reference to canvas with querySelector
📄 src/index.js
const game = gameLoop(GameState);
game.next();
+
+ const canvas = document.querySelector('canvas');
Let's make body full-height
📄 index.html
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Tic Tac Toe</title>
+ <style>
+ html, body {
+ height: 100%;
+ }
+ </style>
</head>
<body>
<script src="./src/index.js" type="module"></script>
and reset default margins
📄 index.html
html, body {
height: 100%;
}
+
+ body {
+ margin: 0;
+ }
</style>
</head>
<body>
Setup canvas size
📄 src/index.js
game.next();
const canvas = document.querySelector('canvas');
+
+ const size = Math.min(document.body.offsetHeight, document.body.offsetWidth);
+ canvas.width = size;
+ canvas.height = size;
and get a 2d context
📄 src/index.js
const size = Math.min(document.body.offsetHeight, document.body.offsetWidth);
canvas.width = size;
canvas.height = size;
+
+ const ctx = canvas.getContext('2d');
Move canvas setup code to separate file
📄 src/canvas-setup.js
export function setupCanvas() {
const canvas = document.querySelector('canvas');
const size = Math.min(document.body.offsetHeight, document.body.offsetWidth);
canvas.width = size;
canvas.height = size;
const ctx = canvas.getContext('2d');
return { canvas, ctx };
}📄 src/index.js
const game = gameLoop(GameState);
game.next();
-
- const canvas = document.querySelector('canvas');
-
- const size = Math.min(document.body.offsetHeight, document.body.offsetWidth);
- canvas.width = size;
- canvas.height = size;
-
- const ctx = canvas.getContext('2d');
and import it to index.js
📄 src/index.js
import { GameState, getWinner, turn } from './game-state.js';
+ import { setupCanvas } from './canvas-setup.js';
function* gameLoop(gameState) {
let winner = -1;
const game = gameLoop(GameState);
game.next();
+
+ const { canvas, ctx } = setupCanvas();
Now let's create render function which will visualize the game state
📄 src/renderer.js
/**
* @typedef GameState
* @property {Number} currentPlayer
* @property {Array<number>} field
*
* @param {HTMLCanvasElement} canvas
* @param {CanvasRenderingContext2D} ctx
* @param {GameState} gameState
*/
export function draw(canvas, ctx, gameState) {
}We'll need to clear the whole canvas on each render call
📄 src/renderer.js
* @param {GameState} gameState
*/
export function draw(canvas, ctx, gameState) {
-
+ ctx.clearRect(0, 0, canvas.width, canvas.height);
}
We'll render each cell with strokeRect, so let's setup cellSize (width and height of each game field cell) and lineWidth (border width of each cell)
📄 src/renderer.js
*/
export function draw(canvas, ctx, gameState) {
ctx.clearRect(0, 0, canvas.width, canvas.height);
+
+ ctx.lineWidth = 10;
+ const cellSize = canvas.width / 3;
+
}
And finally we rendered smth! 🎉
📄 src/renderer.js
ctx.lineWidth = 10;
const cellSize = canvas.width / 3;
+ gameState.field.forEach((_, index) => {
+ const top = Math.floor(index / 3) * cellSize;
+ const left = index % 3 * cellSize;
+
+ ctx.strokeRect(top, left, cellSize, cellSize);
+ });
}
To see the result install live-server
npm i -g live-server
live-server .Wait, what? Nothing rendered 😢
That's because we forgot to import and call draw function
📄 src/index.js
import { GameState, getWinner, turn } from './game-state.js';
import { setupCanvas } from './canvas-setup.js';
+ import { draw } from './renderer.js';
function* gameLoop(gameState) {
let winner = -1;
game.next();
const { canvas, ctx } = setupCanvas();
+ draw(canvas, ctx, GameState);
Let's make canvas a bit smaller to leave some space for other UI
📄 src/canvas-setup.js
export function setupCanvas() {
const canvas = document.querySelector('canvas');
- const size = Math.min(document.body.offsetHeight, document.body.offsetWidth);
+ const size = Math.min(document.body.offsetHeight, document.body.offsetWidth) * 0.8;
canvas.width = size;
canvas.height = size;
and add a css border to make all cell edges look the same
📄 index.html
body {
margin: 0;
}
+
+ canvas {
+ border: 5px solid black;
+ }
</style>
</head>
<body>
It also looks weird in top-left corner, so align canvas to center with flex-box
📄 index.html
<style>
html, body {
height: 100%;
+ display: flex;
+ align-items: center;
+ justify-content: center;
}
body {
So, we've rendered game field cells.
Now let's render X and O symbols
📄 src/renderer.js
ctx.strokeRect(top, left, cellSize, cellSize);
});
}
+
+ /**
+ * @param {CanvasRenderingContext2D} ctx
+ */
+ function drawX(ctx, top, left, size) {
+
+ }
We'll use path to render symbol both for X and O
📄 src/renderer.js
* @param {CanvasRenderingContext2D} ctx
*/
function drawX(ctx, top, left, size) {
+ ctx.beginPath();
+
+ ctx.closePath();
+ ctx.stroke();
}
Draw a line from top-left to bottom-right
📄 src/renderer.js
function drawX(ctx, top, left, size) {
ctx.beginPath();
+ ctx.moveTo(left, top);
+ ctx.lineTo(left + size, top + size);
+
ctx.closePath();
ctx.stroke();
Draw a line from top-right to bottom-left
📄 src/renderer.js
ctx.moveTo(left, top);
ctx.lineTo(left + size, top + size);
+ ctx.moveTo(left + size, top);
+ ctx.lineTo(left, top + size);
+
ctx.closePath();
ctx.stroke();
Rendering O is even more simple
📄 src/renderer.js
ctx.closePath();
ctx.stroke();
+ }
+ /**
+ * @param {CanvasRenderingContext2D} ctx
+ */
+ function drawO(ctx, centerX, centerY, radius) {
+ ctx.beginPath();
+
+ ctx.arc(centerX, centerY, radius, 0, Math.PI * 2);
+ ctx.closePath();
+
+ ctx.stroke();
}
And let's actually render X or O depending on a field value
📄 src/renderer.js
ctx.lineWidth = 10;
const cellSize = canvas.width / 3;
- gameState.field.forEach((_, index) => {
+ gameState.field.forEach((value, index) => {
const top = Math.floor(index / 3) * cellSize;
const left = index % 3 * cellSize;
ctx.strokeRect(top, left, cellSize, cellSize);
+
+ if (value < 0) {
+ return;
+ }
+
+ if (value === 0) {
+ drawX(ctx, top, left, cellSize);
+ } else {
+ drawO(ctx, left + cellSize / 2, top + cellSize / 2, cellSize / 2);
+ }
});
}
Nothing rendered? That's correct, every field value is -2, so let's make some turns
📄 src/index.js
game.next();
const { canvas, ctx } = setupCanvas();
+
+ turn(GameState, 0, 1);
+ turn(GameState, 1, 1);
+ turn(GameState, 2, 0);
+
draw(canvas, ctx, GameState);
📄 src/renderer.js
}
if (value === 0) {
- drawX(ctx, top, left, cellSize);
+ const margin = cellSize * 0.2;
+ const size = cellSize * 0.6;
+
+ drawX(ctx, top + margin, left + margin, size);
} else {
- drawO(ctx, left + cellSize / 2, top + cellSize / 2, cellSize / 2);
+ const radius = cellSize * 0.3;
+ drawO(ctx, left + cellSize / 2, top + cellSize / 2, radius);
}
});
}
Everything seems to be done, the only thing left – interactions. Let's start with cleanup:
📄 src/index.js
const { canvas, ctx } = setupCanvas();
- turn(GameState, 0, 1);
- turn(GameState, 1, 1);
- turn(GameState, 2, 0);
-
draw(canvas, ctx, GameState);
Add click listener and calculate clicked row and col
📄 src/index.js
const { canvas, ctx } = setupCanvas();
draw(canvas, ctx, GameState);
+
+ canvas.addEventListener('click', ({ layerX, layerY }) => {
+ const row = Math.floor(layerY / canvas.height * 100 / 33);
+ const col = Math.floor(layerX / canvas.width * 100 / 33);
+ });
Pass row and col indices to game loop generator
📄 src/index.js
canvas.addEventListener('click', ({ layerX, layerY }) => {
const row = Math.floor(layerY / canvas.height * 100 / 33);
const col = Math.floor(layerX / canvas.width * 100 / 33);
+
+ game.next([row, col]);
});
and reflect game state changes on canvas
📄 src/index.js
const col = Math.floor(layerX / canvas.width * 100 / 33);
game.next([row, col]);
+ draw(canvas, ctx, GameState);
});
Now let's congratulate a winner
📄 src/index.js
winner = getWinner(gameState);
}
+
+ setTimeout(() => {
+ alert(`Congratulations, ${['X', 'O'][winner]}! You won!`);
+ });
}
const game = gameLoop(GameState);
Oh, we forgot to handle a draw! No worries. Let's add isGameFinished helper:
📄 src/game-state.js
return winner;
}
+
+ export function isGameFinished(gameState) {
+ return gameState.field.every(f => f >= 0);
+ }
and call it on each iteration of a game loop
📄 src/index.js
- import { GameState, getWinner, turn } from './game-state.js';
+ import { GameState, getWinner, turn, isGameFinished } from './game-state.js';
import { setupCanvas } from './canvas-setup.js';
import { draw } from './renderer.js';
function* gameLoop(gameState) {
let winner = -1;
- while (winner < 0) {
+ while (winner < 0 && !isGameFinished(gameState)) {
const [rowIndex, colIndex] = yield;
turn(gameState, rowIndex, colIndex);
}
setTimeout(() => {
- alert(`Congratulations, ${['X', 'O'][winner]}! You won!`);
+ if (winner < 0) {
+ alert(`It's a draw`);
+ } else {
+ alert(`Congratulations, ${['X', 'O'][winner]}! You won!`);
+ }
});
}