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 the file byte by byte.
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3 Answers
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));
});
});
4 Comments
WebFreak001
Its utf8 and not utf-8 nodejs.org/api/…
FluffyBeing
What if I just have byte Buffer?
mikep
Pls change
status to error to be compat with doc
Native Dev
Searched for hours to find an example like this. Thank you! I posted a question about the same and then made a synchronous version of this thanks to you. stackoverflow.com/questions/74270008/…
Using @samplebias answer myself, I've discovered that the code could be much simpler + using proper async functionality instead.
const { open } = require('fs/promises');
const path = 'some/dir/your-file.txt' // change this to your local file's location
const start = 0 // place the reading cursor in this position
const size = 100 // following the example from the original answer. chnage this to suit your needs
const handler = await open(path, 'r')
const { buffer, bytesRead } = await handler.read(Buffer.alloc(size), 0, size, start)
const value = buffer.toString('utf-8', 0, bytesRead)
console.log(value)
Comments
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))
});
