html5绘制图片的问题

为什么我把new Image()放在function里面时，图片出不来。
<!DOCTYPE html>
<html>

<head>
    <meta charset="{CHARSET}">
    <title></title>
</head>
<body onload="init()">
    <canvas id="girl"></canvas>
</body>
<script>
    var girl = document.getElementById("girl");
    var girlWidth = 800;
    var girlHeight = 400;
    girl.width = girlWidth;
    girl.height = girlHeight;
    var girlCans = girl.getContext('2d');
    
    
    function init(){
        drawGirl();
    }
    
    function drawGirl(){
        var girlPic = new Image();
        girlPic.src = "img/girl.jpg";
        girlCans.drawImage(girlPic,0,0, girlWidth, girlHeight);
    }
    
    
</script>

</html>

然而我把girlPic设为全局的就能正常显示
<!DOCTYPE html>
<html>

<head>
    <meta charset="{CHARSET}">
    <title></title>
</head>
<body onload="init()">
    <canvas id="girl"></canvas>
</body>
<script>
    var girl = document.getElementById("girl");
    var girlWidth = 800;
    var girlHeight = 400;
    girl.width = girlWidth;
    girl.height = girlHeight;
    var girlCans = girl.getContext('2d');
    var girlPic = new Image();
    girlPic.src = "img/girl.jpg";
    
    function init(){
        drawGirl();
    }
    
    function drawGirl(){
        
        girlCans.drawImage(girlPic,0,0, girlWidth, girlHeight);
    }
    
    
</script>

</html>

因为在image设置完src后，需要一定的时间进行加载，在src设置完后马上运行drawImage函数的时候，image实际上还什么也没有。
比较合适的写法是image.onload = girlCans.drawImage(girlPic,0,0, girlWidth, girlHeight);

img加载是异步滴。需要用onload()

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