Javascript: Detect if there is an image in the file reader

Question

有没有办法查看图像文件是否在javascript的文件读取器中？

Answer 1

You can try to determine if the file is an image by checking few bytes at the beginning of the stream. You can find the header signature of images easily by googling it.

This is a simple way to detect the image type:

 reader.onload = function(e) {
      var buffer = reader.result;
      var int32View = new Int32Array(buffer);
      switch(int32View[0]) {
          case 1196314761: 
              file.verified_type = "image/png";
              break;
          case 944130375:
              file.verified_type = "image/gif";
              break;
          case 544099650:
              file.verified_type = "image/bmp";
              break;
          case -520103681:
              file.verified_type = "image/jpg";
              break;
          default:
                            file.verified_type = "unknown";
              break;
      }
        }; 

You can check few more bytes to give high accuracy to your results.