javascript - JS图片转base64返回字符串

Question

我的代码可以正常工作

function convertImgToBase64URL(url, callback, outputFormat){
    var img = new Image();
    img.crossOrigin = 'Anonymous';
    img.onload = function(){
        var canvas = document.createElement('CANVAS'),
        ctx = canvas.getContext('2d'), dataURL;
        canvas.height = img.height;
        canvas.width = img.width;
        ctx.drawImage(img, 0, 0);
        dataURL = canvas.toDataURL(outputFormat);
        callback(dataURL);
        canvas = null; 
    };
    img.src = url;
}
  
  convertImgToBase64URL('/img/logo.png', function(base64Img){  
    console.log(base64Img);
});

但我需要将结果转换为变量

convertImgToBase64URL('/img/logo.png', function(base64Img){  
   var test = base64Img;
});   
alert(test);

知道如何将结果传递给变量吗？