patrick.net

async function file_type(filepath) {
    const CHUNK_SIZE = 12; // bytes to read at a time; no need to read the rest of the file after the first 12 bytes

    const stream = fs.createReadStream(filepath, { highWaterMark: CHUNK_SIZE });
    for await(const data of stream) { // early return will prevent wasteful reading of entire file
        const bytes = [];
        data.forEach(byte => bytes.push(byte.toString(16)));
        const hex = bytes.join('');

        if (hex.includes('47494638')) return 'gif';
        if (hex.includes('66747970')) return 'mp4';  // magic number for mp4 starts at offset 4
        if (hex.includes('89504e47')) return 'png';
        if (hex.includes('ffd8ff'))   return 'jpg';
        if (hex.includes('57454250')) return 'webp'; // magic number for webp starts at offset 8, goes to 11
        //if (hex.includes('25504446')) return 'pdf';
        //if (hex.includes('504b0304')) return 'zip';
        return false;
    }
}