How to read binary files byte by byte in Node.js

Question

What is the best way to read part of a binary file in Node.js?

I am looking to either access specific bytes in the \"header\" (less than the first 100 bytes) or read

Answer 1

This is another option

const fs = require('fs');

const fileName = 'something.bin'

/*
Requirements ex:
Content in 512 bytes chunks. 
Send the 512 bytes packet as a 1024 char string, where each byte is sent as a 
2 hex digits. 
An "addr" field starts from 0 and tracks the offset of the first byte of the 
packet. 
*/
function chunk(s, maxBytes) {
  //! https://nodejs.org/api/buffer.html#buffer_buf_slice_start_end
  /*
    buf.slice([start[, end]])
    start <integer> Where the new Buffer will start. Default: 0.
    end <integer> Where the new Buffer will end (not inclusive). Default: buf.length.
    Returns: <Buffer>
  */

  let buf = Buffer.from(s);  
  const result = [];
  let readBuffer = true
  let startChunkByte = 0
  let endChunkByte = maxBytes
  while(readBuffer) {
    // First round
    startChunkByte === 0 ? endChunkByte = startChunkByte + maxBytes : ""

    //Handle last chunk
    endChunkByte >= buf.length ? readBuffer = false : ""

    // addr: the position of the first bytes.  raw: the chunk of x bytes
    result.push({"addr":startChunkByte,"raw":buf.slice(startChunkByte, endChunkByte).toString('hex')});

    startChunkByte = endChunkByte
    endChunkByte = startChunkByte + maxBytes
  }
  return result;
}

fs.readFile(fileName, (err, data) => {
  if (err) throw err;
  // Let's choose 512 bytes chunks
  const arrBinChunk =  chunk(data, 512)
  arrBinChunk.forEach(element => console.log(element))

  //Tests - should be able to see 1024, 1024, as per riquerements
  arrBinChunk.forEach(element => console.log(element.raw.length))
});

Answer 2

Here is an example of fs.read()-ing the first 100 bytes from a file descriptor returned by fs.open():

var fs = require('fs');

fs.open('file.txt', 'r', function(status, fd) {
    if (status) {
        console.log(status.message);
        return;
    }
    var buffer = Buffer.alloc(100);
    fs.read(fd, buffer, 0, 100, 0, function(err, num) {
        console.log(buffer.toString('utf8', 0, num));
    });
});