canvas语法提示：
初学canvas时没有语法提示，我们可以通过加入
/** @type {HTMLCanvasElement} */
代码使canvas智能提示，
例如：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<canvas id="cvs" width="500px" height="500px"></canvas>
<script>
/** @type {HTMLCanvasElement} */
let c1 = document.querySelector('#cvs')
let ctx = c1.getContext('2d');
ctx.fillRect(100, 100, 100, 100)
</script>
</body>
</html>