node.js code for embedded data in a png

png-embed.js

const fs = require('fs');
const testPng = fs.readFileSync('input.png');

// ===== crc-32 ==============================================
// https://github.com/SheetJS/js-crc32/blob/master/crc32.js

function signed_crc_table() {
    var c = 0, table = new Array(256);
    for(var n =0; n != 256; ++n){
        c = n;
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
        table[n] = c;
    }
    return typeof Int32Array !== 'undefined' ? new Int32Array(table) : table;
}
var T = signed_crc_table();
function crc32_buf_8(buf) {
    var C = -1, L = buf.length - 7;
    for(var i = 0; i < L;) {
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
        C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
    }
    while(i < L+7) C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
    return C ^ -1;
}

// ===========================================================

const readPNG = buff =>
{
    let i = 8;
    const chunks = [];

    while( i < buff.length )
    {
        const len = buff.readUInt32BE(i);
        i+= 4;

        const type = buff.slice(i,i+4).toString('ascii');
        const typeAndData = buff.slice(i,i+4+len);
        const data = buff.slice( i+4, i+4+len );
        i += 4 + len;

        const crc = buff.readInt32BE(i);
        i += 4;

        const crcGen = crc32_buf_8( typeAndData );

        let textValue = null;
        if( type === 'tEXt' )
        {
            const splitter = data.indexOf( 0 );
            textValue = {
                key: data.slice( 0, splitter ).toString('ascii'),
                value: data.slice( splitter+1, data.length ).toString('ascii'), 
            };

        }

        chunks.push({ len, type, data, typeAndData, crc, crcGen, textValue });
    }

    return chunks;
};

// type Chunk = { text: string, data: Uint8Array }
const writePNG = chunks =>
{
    let result = [];
    
    result.push(Uint8Array.of( 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a ));

    chunks.forEach( chunk =>
    {
        const buff0 = Buffer.alloc( 4 );
        buff0.writeUInt32BE( chunk.data.length, 0 );

        const buff1 = Buffer.alloc( 4 + chunk.data.length );
        buff1.write( chunk.type, 0, 4, 'ascii' );
        chunk.data.copy( buff1, 4, 0, chunk.data.length );

        const buff2 = Buffer.alloc( 4 );
        buff2.writeInt32BE( crc32_buf_8( buff1 ));

        result.push( buff0 );
        result.push( buff1 );
        result.push( buff2 );
    });

    return Buffer.concat(result);
};

const buildTextChunk = ( key, value ) =>
{
    const data = Buffer.alloc( key.length + value.length + 1 );
    data.write( key, 0, 'ascii' );
    data.writeUInt8( 0, key.length );
    data.write( value, key.length + 1, 'ascii' );

    return { type: 'tEXt', data };
};

const buildEndChunk = () => ({ type: 'IEND', data: Buffer.alloc( 0 ) });

const inpng = readPNG( testPng );
const outpng = writePNG( inpng
    .filter( x => x.type !== 'tEXt' && x.type !== 'IEND' )
    .concat([
        buildTextChunk('custom::data', 'hello world!'),
        buildEndChunk()
    ])
);

console.log(inpng);

fs.writeFileSync( 'output.png', outpng );