lib.js

const uint8ArrayUtf8ByteString = (array, start, end) => {
  return String.fromCharCode(...array.slice(start, end));
};

// brands explained: https://github.com/strukturag/libheif/issues/83
// code adapted from: https://github.com/sindresorhus/file-type/blob/6f901bd82b849a85ca4ddba9c9a4baacece63d31/core.js#L428-L438
const isHeic = (buffer) => {
  const brandMajor = uint8ArrayUtf8ByteString(buffer, 8, 12).replace('\0', ' ').trim();

  switch (brandMajor) {
    case 'mif1':
      return true; // {ext: 'heic', mime: 'image/heif'};
    case 'msf1':
      return true; // {ext: 'heic', mime: 'image/heif-sequence'};
    case 'heic':
    case 'heix':
      return true; // {ext: 'heic', mime: 'image/heic'};
    case 'hevc':
    case 'hevx':
      return true; // {ext: 'heic', mime: 'image/heic-sequence'};
  }

  return false;
};

const decodeImage = async (image) => {
  const width = image.get_width();
  const height = image.get_height();

  const { data } = await new Promise((resolve, reject) => {
    image.display({ data: new Uint8ClampedArray(width*height*4), width, height }, (displayData) => {
      if (!displayData) {
        return reject(new Error('HEIF processing error'));
      }

      resolve(displayData);
    });
  });

  return { width, height, data };
};

module.exports = libheif => {
  const decodeBuffer = async ({ buffer, all }) => {
    if (!isHeic(buffer)) {
      throw new TypeError('input buffer is not a HEIC image');
    }

    // wait for module to be initialized
    // currently it is synchronous but it might be async in the future
    await libheif.ready;

    const decoder = new libheif.HeifDecoder();
    const data = decoder.decode(buffer);

    if (!data.length) {
      throw new Error('HEIF image not found');
    }

    if (!all) {
      return await decodeImage(data[0]);
    }

    return data.map(image => {
      return {
        width: image.get_width(),
        height: image.get_height(),
        decode: async () => await decodeImage(image)
      };
    });
  };

  return {
    one: async ({ buffer }) => await decodeBuffer({ buffer, all: false }),
    all: async ({ buffer }) => await decodeBuffer({ buffer, all: true })
  };
};