注：此例子默认图片存放于阿里云CDN节点，本地及其他图片路径待测试。

var imgSrc = "https://img.alicdn.com/bao/uploaded/TB1qimQIpXXXXXbXFXXSutbFXXX.jpg";

//传入图片路径，返回base64
function getBase64(img){
    
    function getBase64Image (img,width,height) {
        var canvas = document.createElement("canvas");
        canvas.width = width ? width : img.width;
        canvas.height = height ? height : img.height;
        var ctx = canvas.getContext("2d");
        ctx.drawImage(img, 0, 0, canvas.width, canvas.height);
        var dataURL = canvas.toDataURL();
        return dataURL;
    }
    var image = new Image();
        // 此属性为解决跨域问题
        image.crossOrigin = '';
        image.src = img;
        var deferred = $.Deferred();
        if (img) {
            image.onload = function () {
                // 将base64传给done上传处理
                deferred.resolve(getBase64Image(image));
            }
            return deferred.promise();
        }
        
}
    
getBase64(imgSrc).then(function(base64){
    console.log(base64);
},function(err){
    console.log(err);
});