HTML5 Snake game implementation ideas and source code_html5 tutorial skills

var canvas; 
var ctx; 
var timer; 
//measures 
var x_cnt = 15; 
var y_cnt = 15; 
var unit = 48; 
var box_x = 0; 
var box_y = 0; 
var box_width = 15 * unit; 
var box_height = 15 * unit; 
var bound_left = box_x; 
var bound_right = box_x + box_width; 
var bound_up = box_y; 
var bound_down = box_y + box_height; 
//images 
var image_sprite; 
//objects 
var snake; 
var food; 
var food_x; 
var food_y; 
//functions 
function Role(sx, sy, sw, sh, direction, status, speed, image, flag) 
{ 
this.x = sx; 
this.y = sy; 
this.w = sw; 
this.h = sh; 
this.direction = direction; 
this.status = status; 
this.speed = speed; 
this.image = image; 
this.flag = flag; 
} 
function transfer(keyCode) 
{ 
switch (keyCode) 
{ 
case 37: 
return 1; 
case 38: 
return 3; 
case 39: 
return 2; 
case 40: 
return 0; 
} 
} 
function addFood() 
{ 
//food_x=box_x+Math.floor(Math.random()*(box_width-unit)); 
//food_y=box_y+Math.floor(Math.random()*(box_height-unit)); 
food_x = unit * Math.floor(Math.random() * x_cnt); 
food_y = unit * Math.floor(Math.random() * y_cnt); 
food = new Role(food_x, food_y, unit, unit, 0, 0, 0, image_sprite, true); 
} 
function play(event) 
{ 
var keyCode; 
if (event == null) 
{ 
keyCode = window.event.keyCode; 
window.event.preventDefault(); 
} 
else 
{ 
keyCode = event.keyCode; 
event.preventDefault(); 
} 
var cur_direction = transfer(keyCode); 
snake[0].direction = cur_direction; 
} 
function update() 
{ 
//add a new part to the snake before move the snake 
if (snake[0].x == food.x && snake[0].y == food.y) 
{ 
var length = snake.length; 
var tail_x = snake[length - 1].x; 
var tail_y = snake[length - 1].y; 
var tail = new Role(tail_x, tail_y, unit, unit, snake[length - 1].direction, 0, 0, image_sprite, true); 
snake.push(tail); 
addFood(); 
} 
//modify attributes 
//move the head 
switch (snake[0].direction) 
{ 
case 0: //down 
snake[0].y += unit; 
if (snake[0].y > bound_down - unit) 
snake[0].y = bound_down - unit; 
break; 
case 1: //left 
snake[0].x -= unit; 
if (snake[0].x < bound_left) 
snake[0].x = bound_left; 
break; 
case 2: //right 
snake[0].x += unit; 
if (snake[0].x > bound_right - unit) 
snake[0].x = bound_right - unit; 
break; 
case 3: //up 
snake[0].y -= unit; 
if (snake[0].y < bound_up) 
snake[0].y = bound_up; 
break; 
} 
//move other part of the snake 
for (var i = snake.length - 1; i >= 0; i--) 
{ 
if (i > 0) 
//snake[i].direction=snake[i-1].direction; 
{ 
snake[i].x = snake[i - 1].x; 
snake[i].y = snake[i - 1].y; 
} 
} 
} 
function drawScene() 
{ 
ctx.clearRect(box_x, box_y, box_width, box_height); 
ctx.strokeStyle = "rgb(0,0,0"; 
ctx.strokeRect(box_x, box_y, box_width, box_height); 
//detection collisions 
//draw images 
for (var i = 0; i < snake.length; i++) 
{ 
ctx.drawImage(image_sprite, snake[i].x, snake[i].y); 
} 
ctx.drawImage(image_sprite, food.x, food.y); 
} 
function init() 
{ 
canvas = document.getElementById("scene"); 
ctx = canvas.getContext(&#39;2d&#39;); 
//images 
image_sprite = new Image(); 
image_sprite.src = "images/sprite.png"; 
image_sprite.onLoad = function () 
{} 
//ojects 
snake = new Array(); 
var head = new Role(0 * unit, 0 * unit, unit, unit, 5, 0, 1, image_sprite, true); 
snake.push(head); 
window.addEventListener(&#39;keydown&#39;, play, false); 
addFood(); 
setInterval(update, 300); //note 
setInterval(drawScene, 30); 
}