canvas drawImage 不显示

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8" />
    <title>Document</title>
</head>
<body>

    <img src="qr.jpg" alt="">
    <canvas width="400px" height="400px">

    </canvas>
    <script>
        var c = document.getElementsByTagName('canvas')[0]
        ctx=c.getContext('2d');

        var img = new Image();
        img.src = "./qr.jpg";
        img.onload = function(){
            ctx.drawImage(img,100,100,240,164);
        }
        // var img=document.getElementsByTagName('img')[0]
        console.log(img)
        // img.src='./bar.png'
    </script>
</body>
</html>

img 要加载完成后才能画到画布上