node.js/ read 100 first bytes of a file

Since node 10 there is the experimental Readable[Symbol.asyncIterator] (that is no more experimental in node v12).

'use strict';

const fs = require('fs');

async function run() {
  const file = 'hello.csv';
  const stream = fs.createReadStream(file, { encoding: 'utf8', start: 0, end: 100 });
  for await (const chunk of stream) {
    console.log(`${file} >>> ${chunk}`);
  }

  // or if you don't want the for-await-loop
  const stream = fs.createReadStream(file, { encoding: 'utf8', start: 0, end: 100 });
  const firstByte = await stream[Symbol.asyncIterator]().next();
  console.log(`${file} >>> ${firstByte.value}`);
}

run();

Will print out the first bites

You're confusing the offset and position argument. From the docs:

offset is the offset in the buffer to start writing at.

position is an integer specifying where to begin reading from in the file. If position is null, data will be read from the current file position.

You should change your code to this:

    fs.read(fd, buffer, 0, contentLength, start, function(err, num) {
        console.log(buffer.toString('utf-8', 0, num));
    });

Basically the offset is will be index that fs.read will write to the buffer. Let's say you have a buffer with length of 10 like this: <Buffer 01 02 03 04 05 06 07 08 09 0a> and you will read from /dev/zero which is basically only zeros, and set the offset to 3 and set the length to 4 then you will get this: <Buffer 01 02 03 00 00 00 00 08 09 0a>.

fs.open('/dev/zero', 'r', function(status, fd) {
    if (status) {
        console.log(status.message);
        return;
    }
    var buffer = new Buffer([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
    fs.read(fd, buffer, 3, 4, 0, function(err, num) {
        console.log(buffer);
    });
});

Also to make things you might wanna try using fs.createStream:

app.post('/random', function(req, res) {
    var start = req.body.start;
    var fileName = './npm';
    var contentLength = req.body.contentlength;
    fs.createReadStream(fileName, { start : start, end: contentLength - 1 })
        .pipe(res);
});