parseOrientation.js

async function parseOrientation(blob) {
  const APP1_MARKER = 0xffe1;
  const EXIF_HEADER = Array.from("Exif")
    .map((c) => c.charCodeAt())
    .reverse()
    .reduce(
      (accumulator, charCode, index) =>
        accumulator + charCode * 16 ** (index * 2)
    );
  const INTEL_BYTE_ALIGN = 0x4949;
  const ORIENTATION_TAG_NO = 0x0112;
  const orientation = {
    value: 9,
    valueOf() {
      return this.value;
    },
    title: undefined,
    toString() {
      return this.title;
    },
  };
  const arrayBuffer = await getArrayBuffer(blob);
  const dataView = {
    exif: null,
    whole: new DataView(arrayBuffer),
  };
  for (let i = 0; i < dataView.whole.byteLength - 4; i++) {
    if (
      dataView.whole.getUint16(i) === APP1_MARKER &&
      dataView.whole.getUint32(i + 4) === EXIF_HEADER
    ) {
      const size = dataView.whole.getUint16(i + 2);
      dataView.exif = new DataView(arrayBuffer.slice(i, size));
      break;
    }
  }
  if (dataView.exif === null) {
    return orientation;
  }
  const littleEndian = dataView.exif.getUint16(10) === INTEL_BYTE_ALIGN;
  for (let i = 0; i < dataView.exif.byteLength - 2; i = i + 2) {
    if (dataView.exif.getUint16(i, littleEndian) === ORIENTATION_TAG_NO) {
      const value = dataView.exif.getUint16(i + 8, littleEndian);
      const title =
        (value === 1 && "upper left") ||
        (value === 3 && "lower right") ||
        (value === 6 && "upper right") ||
        (value === 8 && "lower left") ||
        undefined;
      Object.assign(orientation, {value, title});
      break;
    }
  }
  return orientation;
}

function getArrayBuffer(blob) {
  if ('arrayBuffer' in blob) {
    return blob.arrayBuffer()
  } else {
    return new Promise(resolve => {
      const fileReader = new FileReader()
      fileReader.readAsArrayBuffer(blob)
      fileReader.onloadend = ({ target: { result } }) => resolve(result)
    })
  }
}

😤 Parse blob to extract only orientation and stop parsing.
refer to https://www.media.mit.edu/pia/Research/deepview/exif.html