<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<canvas id="canvas" style="display: block;margin: 0 auto;border: 1px solid #aaa">
</canvas>
<script type="text/javascript">
var searchLight = {x:400,y:300,radius:100,vx:Math.random()*5+10,vy:Math.random()*5+10};
var isIncrease = true;
window.onload = function(){
canvas.width = 800;
canvas.height = 600;
var context = canvas.getContext("2d");
setInterval(
function(){
draw(context);
update();
},
40
)
}
function draw(cxt){
var canvas = cxt.canvas;
cxt.clearRect(0,0,canvas.width,canvas.height);
cxt.save();
cxt.beginPath();
cxt.fillStyle = "black";
cxt.fillRect(0,0,canvas.width,canvas.height);
cxt.save();
cxt.translate(searchLight.x,searchLight.y);
cxt.scale(searchLight.radius,searchLight.radius);
starPath(cxt);
cxt.fillStyle = "#fff";
cxt.fill();
cxt.restore();
cxt.clip();
cxt.font = "bold 150px Arial";
cxt.textAlign = "center";
cxt.textBaseline = "middle";
cxt.fillStyle = "#058";
cxt.fillText("CANVAS",canvas.width/2,canvas.height/4);
cxt.fillText("CANVAS",canvas.width/2,canvas.height/2);
cxt.fillText("CANVAS",canvas.width/2,canvas.height*3/4);
cxt.restore();
}
function update(){
if(searchLight.radius>700){
isIncrease = false
}else if(searchLight.radius<150){
isIncrease = true;
}
if (isIncrease) {
searchLight += 5;
}else{
searchLight -+ 5;
}
}
//绘制星星
function starPath(cxt){
cxt.beginPath();
for (var i = 0; i < 5; i++) {
cxt.lineTo(Math.cos((18 + i*72)/180*Math.PI),
-Math.sin((18 + i*72)/180*Math.PI));
cxt.lineTo(Math.cos((54 + i*72)/180*Math.PI)*0.5,
-Math.sin((54 + i*72)/180*Math.PI)*0.5);
}
cxt.closePath();
}
</script>
</body>
</html>
我发现你一开始没有定义canvas，然后就直接给canvas设置宽高了
function update() {
if (searchLight.radius > 700) {
isIncrease = false
} else if (searchLight.radius < 150) {
isIncrease = true;
}
/* 修改以下部分即可 */
///////////////////////
if (isIncrease) {
searchLight.radius += 5;
} else {
searchLight.radius -= 5;
}
///////////////////////
}