extract_jepg_exif_orientation.js

let offset = 0
  , len = view.byteLength；

// SOI marker
if (view.getUint16(0, false) != 0xFFD8) reject('不是 JPEG 文件');

// APP1 marker
while (offset < len) {
  if (view.getUint16(offset, false) == 0xFFE1) break;
  else offset += 2;
}

if (offset >= len) reject('没找到 APP1 标识');

// now offset point to APP1 marker 0xFFD8
let APP1_offset = offset;

// offset + 4 point offset to EXIF Header
let EXIF_offset = APP1_offset + 4;

// check if  have 'Exif' ascii string: 0x45786966
if (view.getUint32(EXIF_offset, false) != 0x45786966) reject('无 EXIF');

let TIFF_offset = EXIF_offset + 6;

// 0x4d4d: big endian, 0x4949: little endian
let little = view.getUint16(TIFF_offset, false)  == 0x4949 ? true : false

let IFD0_offset = TIFF_offset + view.getUint32(TIFF_offset + 4);

let entries_count =  view.getUint16(IFD0_offset, little);
let entries_offset = IFD0_offset + 2;

for (let i = 0; i < entries_count; i++ ) {
  let tag_offset = entries_offset + (i * 12);
  if (view.getUint16(tag_offset, little) == 0x0112) {
    let resolve_value = view.getUint16(tag_offset + 8, little);
    resolve(resolve_value);
  } 
}
reject('没有 orientation 信息');