js 将图片文件转换成base64

/**
 * 网络图像文件转Base64
 * @param img dom对象 
 */
function getBase64Image(img) {
    var canvas = document.createElement("canvas");
    canvas.width = img.width;
    canvas.height = img.height;
    var ctx = canvas.getContext("2d");
    ctx.drawImage(img, 0, 0, img.width, img.height);
    var ext = img.src.substring(img.src.lastIndexOf(".") + 1).toLowerCase();
    var dataURL = canvas.toDataURL("image/" + ext);
    return dataURL;
}

window.onload = function(){
    // 获取图片对象
    var imgDom = document.getElementById(‘bb‘);
    var dataUrl = getBase64Image(imgDom);
    alert(dataUrl);
}