//将图片转换成base64

        function getBase64Image(url, callback){

          var canvas = document.createElement('canvas'),

              ctx = canvas.getContext('2d'),

              img = new Image();

          //为了解决跨域，可以直接img.crossOrigin=''就能解决图片跨域问题

          img.crossOrigin = 'xes';

          img.onload = function(){

              canvas.height = img.height;

              canvas.width = img.width;

              ctx.drawImage(img,0,0);

              var dataURL = canvas.toDataURL('image/png');

              callback.call(this, dataURL);

              canvas = null;

          }

          img.src = url;

        }

        getBase64Image('http://app1.xesimg.com/roommsg/9071347a804c1b2f9b27ee4198f4e738_small.jpg', function(base64Img){

           console.log(base64Img)

        });