我已经关注了PONG游戏的教程，但是当我在Chrome中打开HTML文件时它不起作用！我是HTML的菜鸟，所以请帮忙！ 有一些评论说每个部分应该做什么。如果我在Chrome中打开了该文件，则会出现此错误：
未捕获的TypeError：无法调用null的“getContext”方法
我也尝试过将代码放入HTML Checker，但它说一切都是正确的！
我的代码：
<!DOCTYPE html>
<html>
<head>
<title>
Pong
</title>
<style type="text/css">
body {
padding: 0;
margin: 0;
overflow: hidden;
}
</style>
<script type="text/javascript">
// RequestAnimFrame: a browser API for getting smooth animations
window.requestAnimFrame = (function(){
return window.requestAnimationFrame ||
window.webkitRequestAnimationFrame ||
window.mozRequestAnimationFrame ||
window.oRequestAnimationFrame ||
window.msRequestAnimationFrame ||
function( callback ){
return window.setTimeout(callback, 1000 / 60);
};
})();
window.cancelRequestAnimFrame = ( function() {
return window.cancelAnimationFrame ||
window.webkitCancelRequestAnimationFrame ||
window.mozCancelRequestAnimationFrame ||
window.oCancelRequestAnimationFrame ||
window.msCancelRequestAnimationFrame ||
clearTimeout
} )();
// Initialize canvas and required variables
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d"), // Create canvas context
W = window.innerWidth, // Window's width
H = window.innerHeight, // Window's height
particles = [], // Array containing particles
ball = {}, // Ball object
paddles = [2], // Array containing two paddles
mouse = {}, // Mouse object to store it's current position
points = 0, // Varialbe to store points
fps = 60, // Max FPS (frames per second)
particlesCount = 20, // Number of sparks when ball strikes the paddle
flag = 0, // Flag variable which is changed on collision
particlePos = {}, // Object to contain the position of collision
multipler = 1, // Varialbe to control the direction of sparks
startBtn = {}, // Start button object
restartBtn = {}, // Restart button object
over = 0, // flag varialbe, cahnged when the game is over
init, // variable to initialize animation
paddleHit;
// Add mousemove and mousedown events to the canvas
canvas.addEventListener("mousemove", trackPosition, true);
canvas.addEventListener("mousedown", btnClick, true);
// Initialise the collision sound
collision = document.getElementById("collide");
// Set the canvas's height and width to full screen
canvas.width = W;
canvas.height = H;
// Function to paint canvas
function paintCanvas() {
ctx.fillStyle = "black";
ctx.fillRect(0, 0, W, H);
}
// Function for creating paddles
function Paddle(pos) {
// Height and width
this.h = 5;
this.w = 150;
// Paddle's position
this.x = W/2 - this.w/2;
this.y = (pos == "top") ? 0 : H - this.h;
}
// Push two new paddles into the paddles[] array
paddles.push(new Paddle("bottom"));
paddles.push(new Paddle("top"));
// Ball object
ball = {
x: 50,
y: 50,
r: 5,
c: "white",
vx: 4,
vy: 8,
// Function for drawing ball on canvas
draw: function() {
ctx.beginPath();
ctx.fillStyle = this.c;
ctx.arc(this.x, this.y, this.r, 0, Math.PI*2, false);
ctx.fill();
}
};
// Start Button object
startBtn = {
w: 100,
h: 50,
x: W/2 - 50,
y: H/2 - 25,
draw: function() {
ctx.strokeStyle = "white";
ctx.lineWidth = "2";
ctx.strokeRect(this.x, this.y, this.w, this.h);
ctx.font = "18px Arial, sans-serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillStlye = "white";
ctx.fillText("Start", W/2, H/2 );
}
};
// Restart Button object
restartBtn = {
w: 100,
h: 50,
x: W/2 - 50,
y: H/2 - 50,
draw: function() {
ctx.strokeStyle = "white";
ctx.lineWidth = "2";
ctx.strokeRect(this.x, this.y, this.w, this.h);
ctx.font = "18px Arial, sans-serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillStlye = "white";
ctx.fillText("Restart", W/2, H/2 - 25 );
}
};
// Function for creating particles object
function createParticles(x, y, m) {
this.x = x || 0;
this.y = y || 0;
this.radius = 1.2;
this.vx = -1.5 + Math.random()*3;
this.vy = m * Math.random()*1.5;
}
// Draw everything on canvas
function draw() {
paintCanvas();
for(var i = 0; i < paddles.length; i++) {
p = paddles[i];
ctx.fillStyle = "white";
ctx.fillRect(p.x, p.y, p.w, p.h);
}
ball.draw();
update();
}
// Function to increase speed after every 5 points
function increaseSpd() {
if(points % 4 == 0) {
if(Math.abs(ball.vx) < 15) {
ball.vx += (ball.vx < 0) ? -1 : 1;
ball.vy += (ball.vy < 0) ? -2 : 2;
}
}
}
// Track the position of mouse cursor
function trackPosition(e) {
mouse.x = e.pageX;
mouse.y = e.pageY;
}
// Function to update positions, score and everything.
// Basically, the main game logic is defined here
function update() {
// Update scores
updateScore();
// Move the paddles on mouse move
if(mouse.x && mouse.y) {
for(var i = 1; i < paddles.length; i++) {
p = paddles[i];
p.x = mouse.x - p.w/2;
}
}
// Move the ball
ball.x += ball.vx;
ball.y += ball.vy;
// Collision with paddles
p1 = paddles[1];
p2 = paddles[2];
// If the ball strikes with paddles,
// invert the y-velocity vector of ball,
// increment the points, play the collision sound,
// save collision's position so that sparks can be
// emitted from that position, set the flag variable,
// and change the multiplier
if(collides(ball, p1)) {
collideAction(ball, p1);
}
else if(collides(ball, p2)) {
collideAction(ball, p2);
}
else {
// Collide with walls, If the ball hits the top/bottom,
// walls, run gameOver() function
if(ball.y + ball.r > H) {
ball.y = H - ball.r;
gameOver();
}
else if(ball.y < 0) {
ball.y = ball.r;
gameOver();
}
// If ball strikes the vertical walls, invert the
// x-velocity vector of ball
if(ball.x + ball.r > W) {
ball.vx = -ball.vx;
ball.x = W - ball.r;
}
else if(ball.x -ball.r < 0) {
ball.vx = -ball.vx;
ball.x = ball.r;
}
}
// If flag is set, push the particles
if(flag == 1) {
for(var k = 0; k < particlesCount; k++) {
particles.push(new createParticles(particlePos.x, particlePos.y, multiplier));
}
}
// Emit particles/sparks
emitParticles();
// reset flag
flag = 0;
}
//Function to check collision between ball and one of
//the paddles
function collides(b, p) {
if(b.x + ball.r >= p.x && b.x - ball.r <=p.x + p.w) {
if(b.y >= (p.y - p.h) && p.y > 0){
paddleHit = 1;
return true;
}
else if(b.y <= p.h && p.y == 0) {
paddleHit = 2;
return true;
}
else return false;
}
}
//Do this when collides == true
function collideAction(ball, p) {
ball.vy = -ball.vy;
if(paddleHit == 1) {
ball.y = p.y - p.h;
particlePos.y = ball.y + ball.r;
multiplier = -1;
}
else if(paddleHit == 2) {
ball.y = p.h + ball.r;
particlePos.y = ball.y - ball.r;
multiplier = 1;
}
points++;
increaseSpd();
if(collision) {
if(points > 0)
collision.pause();
collision.currentTime = 0;
collision.play();
}
particlePos.x = ball.x;
flag = 1;
}
// Function for emitting particles
function emitParticles() {
for(var j = 0; j < particles.length; j++) {
par = particles[j];
ctx.beginPath();
ctx.fillStyle = "white";
if (par.radius > 0) {
ctx.arc(par.x, par.y, par.radius, 0, Math.PI*2, false);
}
ctx.fill();
par.x += par.vx;
par.y += par.vy;
// Reduce radius so that the particles die after a few seconds
par.radius = Math.max(par.radius - 0.05, 0.0);
}
}
// Function for updating score
function updateScore() {
ctx.fillStlye = "white";
ctx.font = "16px Arial, sans-serif";
ctx.textAlign = "left";
ctx.textBaseline = "top";
ctx.fillText("Score: " + points, 20, 20 );
}
// Function to run when the game overs
function gameOver() {
ctx.fillStlye = "white";
ctx.font = "20px Arial, sans-serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";
ctx.fillText("Game Over - You scored "+points+" points!", W/2, H/2 + 25 );
// Stop the Animation
cancelRequestAnimFrame(init);
// Set the over flag
over = 1;
// Show the restart button
restartBtn.draw();
}
// Function for running the whole animation
function animloop() {
init = requestAnimFrame(animloop);
draw();
}
// Function to execute at startup
function startScreen() {
draw();
startBtn.draw();
}
// On button click (Restart and start)
function btnClick(e) {
// Variables for storing mouse position on click
var mx = e.pageX,
my = e.pageY;
// Click start button
if(mx >= startBtn.x && mx <= startBtn.x + startBtn.w) {
animloop();
// Delete the start button after clicking it
startBtn = {};
}
// If the game is over, and the restart button is clicked
if(over == 1) {
if(mx >= restartBtn.x && mx <= restartBtn.x + restartBtn.w) {
ball.x = 20;
ball.y = 20;
points = 0;
ball.vx = 4;
ball.vy = 8;
animloop();
over = 0;
}
}
}
// Show the start screen
startScreen();
</script>
</head>
<body>
<canvas id="canvas"></canvas>
</body>
</html>
答案 0 :(得分：2)
这是因为您的脚本运行：
var canvas = document.getElementById("canvas"),
ctx = canvas.getContext("2d"), // Create canvas context
之前canvas元素存在：
<canvas id="canvas"></canvas>
将该部分移动到HTML的底部，事情应该会更好。
<body>
<canvas id="canvas"></canvas>
<script>
<!-- Move all your script here -->
</script>
</body>