img通过canvas转成base64编码

<script type="text/javascript">
    function getBase64Image(img) {
      var canvas = document.createElement("canvas");
      canvas.width = img.width;
      canvas.height = img.height;

      var ctx = canvas.getContext("2d");
      ctx.drawImage(img, , , img.width, img.height);

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

      // return dataURL.replace("data:image/png;base64,", "");
    }

    function main() {
      var img = document.createElement('img');
      img.src = './images/yeoman.png';
      img.onload =function() {
        var data = getBase64Image(img);
        console.log(data);
      }

      document.body.appendChild(img);
    }

    main()

  </script>