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

There's a built-in module almost for this case - readline. It avoids messing with chunks and so forth. The code would look like the following:

const fs = require('fs');const readline = require('readline');async function getFirstLine(pathToFile) {  const readable = fs.createReadStream(pathToFile);  const reader = readline.createInterface({ input: readable });  const line = await new Promise((resolve) => {    reader.on('line', (line) => {      reader.close();      resolve(line);    });  });  readable.close();  return line;}

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()!

I know this doesn't exactly answer the question but for those who are looking for a READABLE and simple way to do so:

const fs = require('fs').promises;async function getFirstLine(filePath) {    const fileContent = await fs.readFile(filePath, 'utf-8');    return (fileContent.match(/(^.*)/) || [])[1] || '';} 

NOTE:

• naturaly, this will only work with text files, which I assumed you used from your description
• this will work with empty files and will return an empty string
• this regexp is very performant since it is simple (no OR conditions`or complex matches) and only reads the first line