canvas将图片转成base64格式 以及 验证图片是否透明

logoImgUpload:function(file) {
        let self = this;
        self.formatUpload(file);
        let reader = new FileReader();
        reader.readAsDataURL(file);
        reader.onload = function (e) {
            var img = new Image();
            img.src = e.target.result;
            self.isAlphaBackground = false;
            // 缩放图片需要的canvas
            var canvas = document.createElement('canvas');
            var context = canvas.getContext('2d');

            // base64地址图片加载完毕后
            img.onload = function () {
                // 图片原始尺寸
                var originWidth = this.width;
                var originHeight = this.height;
                // canvas尺寸设置
                canvas.width = originWidth;
                canvas.height = originHeight;
                // 清除画布
                context.clearRect(0, 0, originWidth, originHeight);
                // 图片绘制在画布上
                context.drawImage(img, 0, 0);
                // 获取图片像素信息
                var imageData = context.getImageData(0, 0, originWidth, originHeight).data;
                // 检测有没有透明数据
                self.isAlphaBackground = false;
                for (var index = 3; index < imageData.length; index += 4) {
                    if (imageData[index] != 255) {
                        self.isAlphaBackground = true;
                        break;    
                    }
                }
                // isAlphaBackground就是最后石头有透明或半透明背景色的结果
                console.log(self.isAlphaBackground)
            };
            self.design.logo = e.target.result;
        }
        return false;
    },