Get exit orientation from a file using JavaScript and TypeScript

calculate-orientation.ts

enum Bytes {
  JPGStartMarker = 0xffd8,
  EXIFMarker = 0xffe1,
  EXIFInfo = 0x45786966,
  OrientationMarker = 0x0112,
  Empty = 0xff00,
  ExifAlignment = 0x4949
}

/**
 * Checks given file for `orientation` exif value.
 * More info: https://www.media.mit.edu/pia/Research/deepview/exif.html
 *
 * @param file File which exif is being checked
 * @returns Promise<number> where number represents image orientation as explained below on capital letter F

    1        2       3      4         5            6           7          8

    ██████  ██████      ██  ██      ██████████  ██                  ██  ██████████
    ██          ██      ██  ██      ██  ██      ██  ██          ██  ██      ██  ██
    ████      ████    ████  ████    ██          ██████████  ██████████          ██
    ██          ██      ██  ██
    ██          ██  ██████  ██████

 */
export function calculateOrientation(file: File): Promise<number> {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();

    reader.readAsArrayBuffer(file.slice(0, 64 * 1024));

    reader.onerror = reject;

    // https://stackoverflow.com/questions/35789498/new-typescript-1-8-4-build-error-build-property-result-does-not-exist-on-t
    reader.onload = (event: any) => {
      const view = new DataView(event.target.result);

      if (view.getUint16(0, false) !== Bytes.JPGStartMarker) {
        // skip if file does not start with JPG start marker. Probably not a jpg.
        resolve(-2);
      }
      const length = view.byteLength;
      let offset = 2;

      while (offset < length) {
        const marker = view.getUint16(offset, false);
        offset += 2;

        if (marker === Bytes.EXIFMarker) {
          offset += 2;

          if (view.getUint32(offset, false) !== Bytes.EXIFInfo) {
            // no exif info found in exif marker.
            resolve(-1);
          }

          // check for bytes align. Data can be stored at different addresses (Intel, Motorola standards), so we need to make sure to get correct one.
          const little = view.getUint16((offset += 6), false) === Bytes.ExifAlignment;

          offset += view.getUint32(offset + 4, little);

          // Get the exif tags count and go through all the elements.
          const exifTags = view.getUint16(offset, little);
          offset += 2;

          for (let i = 0; i < exifTags; i += 1) {
            // Get the tag and check if its correct one.
            const exifOffset = offset + i * 12;
            if (view.getUint16(exifOffset, little) === Bytes.OrientationMarker) {
              // we're only interested in value. Skip name and length markers.
              resolve(view.getUint16(exifOffset + 8, little));
            }
          }
          // eslint-disable-next-line no-bitwise
        } else if ((marker & Bytes.Empty) !== Bytes.Empty) {
          // Empty marker. Skipping.
          break;
        } else {
          offset += view.getUint16(offset, false); // skip current marker if exif marker not found
        }
      }
      resolve(-1);
    };
  });
}