<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<title>canvas五角星</title>
<script>
window.onload = function () {
let canvas = document.getElementById('canvas');
let context = canvas.getContext('2d');
draw(context, 150, 300, 400, 400);
}
function draw(context, r, R, x, y) {
context.beginPath();
for (let i = 0; i < 5; i++) {
context.lineTo((Math.cos((18 + i * 72) / 180 * Math.PI)) * R + x, -(Math.sin((18 + i * 72) / 180 * Math.PI)) * R + y);
context.lineTo((Math.cos((54 + i * 72) / 180 * Math.PI)) * r + x, -(Math.sin((54 + i * 72) / 180 * Math.PI)) * r + y);
}
context.lineWidth = 10;
context.closePath();
context.strokeStyle = 'red';
context.fillStyle = '#123333';
context.stroke();
}
</script>
</head>
<body style="height:100%">
<canvas id="canvas" style="height:800px;width:800px;display:block;margin:20px auto;border:1px solid #666">抱歉，您的浏览器不支持canvas，请更换其他浏览器！</canvas>
</body>
</html>
canvas的长宽不能加px
canvas的宽高不能写在style里面，写在外面或者用js来控制。
<canvas id="canvas" width="800" height="800" style="display:block;margin:20px auto;border:1px solid #666">抱歉，您的浏览器不支持canvas，请更换其他浏览器！</canvas>
或者canvas标签里面不指定宽高，在js里面写canvas.width=800;canvas.height=800;
onload函数中加入canvas.width=800;canvas.height=800;