In Node.js how do I change (overwrite) a byte in a binary file without adding bytes

Okay, I figured out it is actually pretty straight forward ^^

fs.open(filename, "r+", (err, fd) => {
    if(!err) {
        fs.write(
            fd, new Uint8Array([byte]), 0, 1, offset,
            (err, bw, buf) => {
                if(!err) {
                    // succesfully wrote byte to offset
                }
            }
        );
    }
});

I had to do something like what you described recently. I had to update a URL in an executable without changing the size. The key is to use a Transform on a stream. The idea is that the Transform will read in and write out the exact data you want and only modify the bytes you specify.

Here's a Transform class that does a find and replace within a stream. The constructor takes arguments for what the start and end sequences of bytes are for the chunk that should be replaced. There is also a padValue argument that is used to maintain that same size.

import { Transform } from 'stream'

export default class FindAndReplaceTransform extends Transform {
  constructor(startBuffer, endBuffer, replacementValueBuffer, padValue, options) {
    super(options);
    this.startBuffer = startBuffer;
    this.endBuffer = endBuffer;
    this.replacementValueBuffer = replacementValueBuffer;
    this.padValue = padValue;
  }

  _findInBuffer(sourceBuffer, searchBuffer) {
    let searchFound = -1;
    let lengthOfPartialMatch = 0;
    for (let i = 0; i < sourceBuffer.length; i++) {
      for (let j = 0; j < searchBuffer.length; j++) {
        if (i + j >= sourceBuffer.length) {
          if (j > 0) {
            lengthOfPartialMatch = j;
          }
          break;
        }
        if (sourceBuffer[i + j] !== searchBuffer[j]) {
          break;
        }
        if (j === searchBuffer.length - 1) {
          searchFound = i;
        }
      }
      if (searchFound >= 0 || lengthOfPartialMatch > 0) {
        break;
      }
    }
    return { searchFound, lengthOfPartialMatch };
  }

  _doReplacement(length) {
    let replacementValueBuffer = this.replacementValueBuffer;
    if (this.padValue !== undefined) {
      replacementValueBuffer = Buffer.concat([replacementValueBuffer, Buffer.alloc(length - replacementValueBuffer.length, this.padValue)], length);
    }
    this.push(replacementValueBuffer);
  }

  //override
  _transform(data, encoding, done) {
    if(this.lengthOfPartialStartMatch){
      data = Buffer.concat([this.startBuffer.slice(0, this.lengthOfPartialStartMatch), data], this.lengthOfPartialStartMatch + data.length);
      delete this.lengthOfPartialStartMatch;
    }
    if(this.lengthOfPartialEndMatch){
      data = Buffer.concat([this.endBuffer.slice(0, this.lengthOfPartialEndMatch), data], this.lengthOfPartialEndMatch + data.length);
      this.replacementBuffer = this.replacementBuffer.slice(0, this.replacementBuffer.length - this.lengthOfPartialEndMatch);
      delete this.lengthOfPartialEndMatch;
    }

    let startAlreadyFound = !!this.replacementBuffer
    let { searchFound: startIndex, lengthOfPartialMatch: lengthOfPartialStartMatch } = this._findInBuffer(data, this.startBuffer);
    let tail = data.slice(startIndex >= 0 && !startAlreadyFound ? startIndex : 0);
    let { searchFound: endIndex, lengthOfPartialMatch: lengthOfPartialEndMatch } = this._findInBuffer(tail, this.endBuffer);


    if (!startAlreadyFound && startIndex >= 0) {
      this.push(data.slice(0, startIndex))
      this.replacementBuffer = Buffer.alloc(0);
      startAlreadyFound = true;
    }
    if (startAlreadyFound) {
      if (endIndex >= 0) {
        let replacementLength = this.replacementBuffer.length + endIndex + this.endBuffer.length;
        this._doReplacement(replacementLength);
        delete this.replacementBuffer;
        if (endIndex + this.endBuffer.length < tail.length) {
          let remainder = tail.slice(endIndex + this.endBuffer.length)
          this._transform(remainder, encoding, done);
          return;
        }
      } else {
        this.lengthOfPartialEndMatch = lengthOfPartialEndMatch;
        this.replacementBuffer = Buffer.concat([this.replacementBuffer, tail], this.replacementBuffer.length + tail.length);
      }
    } else {
      this.lengthOfPartialStartMatch = lengthOfPartialStartMatch;
      this.push(data.slice(0, data.length - lengthOfPartialStartMatch))
    }
    done();
  }

  //override
  _flush(done) {
    if (this.replacementBuffer) {
      this.push(this.replacementBuffer)
    }
    if(this.lengthOfPartialStartMatch){
      this.push(this.startBuffer.slice(0, this.lengthOfPartialStartMatch));
    }
    delete this.replacementBuffer;
    delete this.lengthOfPartialStartMatch;
    delete this.lengthOfPartialEndMatch;
    done()
  }
}

To use the above transform, you could do something like this:

let stream = fs.createReadStream(inputFile);
let padding = 0x00;
let startSequence = Buffer.from('${', 'utf16le');
let endSequence = Buffer.from('}', 'utf16le');
let transform = new FindAndReplaceTransform(startSequence, endSequence, Buffer.from(replacementValue, 'utf16le'), paddingValue);
stream = stream.pipe(transform);
stream.pipe(fs.createWriteStream(outputFile));

Obviously, if all you want to do is change a byte at a certain offset, the Transform class will be significantly simpler. I provided that above code because I had it and if you want to do something a little more complex, you have it for reference.

The main method that you want to make sure to implement is the _transform method. You also may need to implement the _flush method depending on you implementation. The other class methods in the above code are for my implementation of the replacement code and are not needed for a Transform to work.