What is the most efficient way to read only the first line of a file in Node JS?

Question

Imagine you have many long text files, and you need to only extract data from the first line of each one (without reading any further content). What is the best way in Node JS to do it?

Thanks!

Answer 1

I ended up adopting this solution, which seems the most performant I've seen so far:

var fs = require('fs');
var Q = require('q');

function readFirstLine (path) {
  return Q.promise(function (resolve, reject) {
    var rs = fs.createReadStream(path, {encoding: 'utf8'});
    var acc = '';
    var pos = 0;
    var index;
    rs
      .on('data', function (chunk) {
        index = chunk.indexOf('\n');
        acc += chunk;
        index !== -1 ? rs.close() : pos += chunk.length;
      })
      .on('close', function () {
        resolve(acc.slice(0, pos + index));
      })
      .on('error', function (err) {
        reject(err);
      })
  });
}

I created a npm module for convenience, named "firstline".

Thanks to @dandavis for the suggestion to use String.prototype.slice()!