The straightforward solution would be:

1. read file
2. split by \n, save to array
3. get number of lines (== length of array)
4. generate random number between 0 and number of lines
5. return array[generated_number]

But it might use quite a bit of memory if your file is really large.

so without reading the entire file, off the top of my head - Maybe seek to a random position, then read until you hit \n, and then again read until \n, then split by \n and pick the middle part of the array?

Note- this will break if the last line isn't followed by a new line. (if the random position is in the last two lines)

two approaches here:

1. read all file, choose random number between 1 and 5000 and return that line (not good for general solution)
2. pre-define random line and return it when getting to that line.

const readline = require("readline");
const fs = require("fs");

const file = readline.createInterface({
    input: fs.createReadStream('file.txt'),
    output: process.stdout,
    terminal: false
});
const radnomLineNumber = getRandomInt(5000);
let i = 0;
file.on('line', (line) => {
   if (randomLineNumber === i) {
      console.log(line);
      return;
   }
   i++;
});