<!DOCTYPE html>

<html>

<head>

<title>Basic Snake HTML Game</title>

<style>

html, body {

height: 100%;

margin: 0;

}

body {

background: black;

display: flex;

align-items: center;

justify-content: center;

}

canvas {

border: 1px solid white;

}

</style>

</head>

<body>

var canvas = document.getElementById('game');

var context = canvas.getContext('2d');

// the canvas width & height, snake x & y, and the apple x & y, all need to be a multiples of the grid size in order for collision detection to work

// (e.g. 16 * 25 = 400)

var grid = 16;

var count = 0;

var snake = {

x: 160,

y: 160,

// snake velocity. moves one grid length every frame in either the x or y direction

dx: grid,

dy: 0,

// keep track of all grids the snake body occupies

cells: [],

// length of the snake. grows when eating an apple

maxCells: 4

};

var apple = {

x: 320,

y: 320

};

// get random whole numbers in a specific range

// @see https://stackoverflow.com/a/1527820/2124254

function getRandomInt(min, max) {

return Math.floor(Math.random() * (max - min)) + min;

}

// game loop

function loop() {

requestAnimationFrame(loop);

// slow game loop to 15 fps instead of 60 (60/15 = 4)

if (++count < 4) {

return;

}

count = 0;

context.clearRect(0,0,canvas.width,canvas.height);

// move snake by it's velocity

snake.x += snake.dx;

snake.y += snake.dy;

// wrap snake position horizontally on edge of screen

if (snake.x < 0) {

snake.x = canvas.width - grid;

}

else if (snake.x >= canvas.width) {

snake.x = 0;

}

// wrap snake position vertically on edge of screen

if (snake.y < 0) {

snake.y = canvas.height - grid;

}

else if (snake.y >= canvas.height) {

snake.y = 0;

}

// keep track of where snake has been. front of the array is always the head

snake.cells.unshift({x: snake.x, y: snake.y});

// remove cells as we move away from them

if (snake.cells.length > snake.maxCells) {

snake.cells.pop();

}

// draw apple

context.fillStyle = 'red';

context.fillRect(apple.x, apple.y, grid-1, grid-1);

// draw snake one cell at a time

context.fillStyle = 'green';

snake.cells.forEach(function(cell, index) {

// drawing 1 px smaller than the grid creates a grid effect in the snake body so you can see how long it is

context.fillRect(cell.x, cell.y, grid-1, grid-1);

// snake ate apple

if (cell.x === apple.x && cell.y === apple.y) {

snake.maxCells++;

// canvas is 400x400 which is 25x25 grids

apple.x = getRandomInt(0, 25) * grid;

apple.y = getRandomInt(0, 25) * grid;

}

// check collision with all cells after this one (modified bubble sort)

for (var i = index + 1; i < snake.cells.length; i++) {

// snake occupies same space as a body part. reset game

if (cell.x === snake.cells[i].x && cell.y === snake.cells[i].y) {

snake.x = 160;

snake.y = 160;

snake.cells = [];

snake.maxCells = 4;

snake.dx = grid;

snake.dy = 0;

apple.x = getRandomInt(0, 25) * grid;

apple.y = getRandomInt(0, 25) * grid;

}

}

});

}

// listen to keyboard events to move the snake

document.addEventListener('keydown', function(e) {

// prevent snake from backtracking on itself by checking that it's

// not already moving on the same axis (pressing left while moving

// left won't do anything, and pressing right while moving left

// shouldn't let you collide with your own body)

// left arrow key

if (e.which === 37 && snake.dx === 0) {

snake.dx = -grid;

snake.dy = 0;

}

// up arrow key

else if (e.which === 38 && snake.dy === 0) {

snake.dy = -grid;

snake.dx = 0;

}

// right arrow key

else if (e.which === 39 && snake.dx === 0) {

snake.dx = grid;

snake.dy = 0;

}

// down arrow key

else if (e.which === 40 && snake.dy === 0) {

snake.dy = grid;

snake.dx = 0;

}

});

// start the game

requestAnimationFrame(loop);

</script>

</body>

</html>