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