A simplified JPEG orientation checker

orientation.js

/* Based off https://github.com/mattiasw/ExifReader */

(function() {
  (typeof exports !== "undefined" && exports !== null ? exports : this).ExifReader = (function() {
    ExifReader.prototype._MIN_DATA_BUFFER_LENGTH = 2;
    ExifReader.prototype._JPEG_ID_SIZE = 2;
    ExifReader.prototype._JPEG_ID = 0xffd8;
    ExifReader.prototype._APP_MARKER_SIZE = 2;
    ExifReader.prototype._APP0_MARKER = 0xffe0;
    ExifReader.prototype._APP1_MARKER = 0xffe1;
    ExifReader.prototype._APP15_MARKER = 0xffef;
    ExifReader.prototype._APP_ID_OFFSET = 4;
    ExifReader.prototype._BYTES_Exif = 0x45786966;
    ExifReader.prototype._TIFF_HEADER_OFFSET = 10;
    ExifReader.prototype._BYTE_ORDER_BIG_ENDIAN = 0x4949;
    ExifReader.prototype._BYTE_ORDER_LITTLE_ENDIAN = 0x4d4d;
    ExifReader.prototype._ORIENTATION_TAG_CODE = 274;

    function ExifReader() {
      var _this = this;
      this._tiffHeaderOffset = 0;
    }

    ExifReader.prototype.getOrientation = function(data) {
      this._dataView = new DataView(data);
      this._checkImageHeader();
      this._readTags();
      this._dataView = null;
      return this.orientation;
    };

    ExifReader.prototype._checkImageHeader = function() {
      if (this._dataView.byteLength < this._MIN_DATA_BUFFER_LENGTH || this._dataView.getUint16(0, false) !== this._JPEG_ID) {
        throw new Error('Invalid image format');
      }
      this._parseAppMarkers(this._dataView);
      if (!this._hasExifData()) {
        throw new Error('No Exif data');
      }
    };

    ExifReader.prototype._parseAppMarkers = function(dataView) {
      var appMarkerPosition, fieldLength, _results;

      appMarkerPosition = this._JPEG_ID_SIZE;
      _results = [];
      while (true) {
        if (dataView.byteLength < appMarkerPosition + this._APP_ID_OFFSET + 5) {
          break;
        }
        if (this._isApp1ExifMarker(dataView, appMarkerPosition)) {
          fieldLength = dataView.getUint16(appMarkerPosition + this._APP_MARKER_SIZE, false);
          this._tiffHeaderOffset = appMarkerPosition + this._TIFF_HEADER_OFFSET;
        } else if (this._isAppMarker(dataView, appMarkerPosition)) {
          fieldLength = dataView.getUint16(appMarkerPosition + this._APP_MARKER_SIZE, false);
        } else {
          break;
        }
        _results.push(appMarkerPosition += this._APP_MARKER_SIZE + fieldLength);
      }
      return _results;
    };

    ExifReader.prototype._isApp1ExifMarker = function(dataView, appMarkerPosition) {
      return dataView.getUint16(appMarkerPosition, false) === this._APP1_MARKER && dataView.getUint32(appMarkerPosition + this._APP_ID_OFFSET, false) === this._BYTES_Exif && dataView.getUint8(appMarkerPosition + this._APP_ID_OFFSET + 4, false) === 0x00;
    };

    ExifReader.prototype._isAppMarker = function(dataView, appMarkerPosition) {
      var appMarker;

      appMarker = dataView.getUint16(appMarkerPosition, false);
      return appMarker >= this._APP0_MARKER && appMarker <= this._APP15_MARKER;
    };

    ExifReader.prototype._hasExifData = function() {
      return this._tiffHeaderOffset !== 0;
    };

    ExifReader.prototype._readTags = function() {
      this._setByteOrder();
      this._readIfd(this._getIfdOffset());
    };

    ExifReader.prototype._setByteOrder = function() {
      if (this._dataView.getUint16(this._tiffHeaderOffset) === this._BYTE_ORDER_BIG_ENDIAN) {
        this._littleEndian = true;
      } else if (this._dataView.getUint16(this._tiffHeaderOffset) === this._BYTE_ORDER_LITTLE_ENDIAN) {
        this._littleEndian = false;
      } else {
        throw new Error('Illegal byte order value. Faulty image.');
      }
    };

    ExifReader.prototype._getIfdOffset = function() {
      return this._tiffHeaderOffset + this._dataView.getUint32(this._tiffHeaderOffset + 4);
    };

    ExifReader.prototype._readIfd = function(offset) {
      var fieldIndex, numberOfFields, tag, _i;

      numberOfFields = this._dataView.getUint16(offset, this._littleEndian);
      offset += 2;
      for (fieldIndex = _i = 0; 0 <= numberOfFields ? _i < numberOfFields : _i > numberOfFields; fieldIndex = 0 <= numberOfFields ? ++_i : --_i) {
        tag = this._readTag(offset);
        if (tag) {
          this.orientation = tag;
          break;
        }
        offset += 12;
      }
      return tag;
    };

    ExifReader.prototype._readTag = function(offset) {
      var tagCode, value;

      tagCode = this._dataView.getUint16(offset, this._littleEndian);

      if (tagCode != this._ORIENTATION_TAG_CODE) {
        return;
      }
      value = this._dataView.getUint16(offset + 8, this._littleEndian);
      return this._orientationName(value);
    };

    ExifReader.prototype._orientationName = function(value) {
      switch (value) {
        case 1:
          return 'top-left';
        case 2:
          return 'top-right';
        case 3:
          return 'bottom-right';
        case 4:
          return 'bottom-left';
        case 5:
          return 'left-top';
        case 6:
          return 'right-top';
        case 7:
          return 'right-bottom';
        case 8:
          return 'left-bottom';
        default:
          return 'Undefined';
      }
    };

    return ExifReader;

  })();

}).call(this);