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

Question

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

Thanks!

Answer 1

In the end, I decided to use this solution, which seems to be the most perfect 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 an npm module for convenience called " firstline ."

Thanks to @dandavis for suggesting using String.prototype.slice() !

Answer 2

// Here you go;

 var lineReader = require('line-reader'); var async = require('async'); exports.readManyFiles = function(files) { async.map(files, function(file, callback)) lineReader.open(file, function(reader) { if (reader.hasNextLine()) { reader.nextLine(function(line) { callback(null,line); }); } }); }, function(err, allLines) { //do whatever you want to with the lines }) } 

Answer 3

Please try the following:

https://github.com/yinrong/node-line-stream-util#get-head-lines

The upstream problem after it received the headers.