Parsing huge logfiles in Node.js - read in line-by-line

You can use the inbuilt readline package, see docs here. I use stream to create a new output stream.

var fs = require('fs'),
    readline = require('readline'),
    stream = require('stream');

var instream = fs.createReadStream('/path/to/file');
var outstream = new stream;
outstream.readable = true;
outstream.writable = true;

var rl = readline.createInterface({
    input: instream,
    output: outstream,
    terminal: false
});

rl.on('line', function(line) {
    console.log(line);
    //Do your stuff ...
    //Then write to outstream
    rl.write(cubestuff);
});

Large files will take some time to process. Do tell if it works.

I searched for a solution to parse very large files (gbs) line by line using a stream. All the third-party libraries and examples did not suit my needs since they processed the files not line by line (like 1 , 2 , 3 , 4 ..) or read the entire file to memory

The following solution can parse very large files, line by line using stream & pipe. For testing I used a 2.1 gb file with 17.000.000 records. Ram usage did not exceed 60 mb.

First, install the event-stream package:

npm install event-stream

Then:

var fs = require('fs')
    , es = require('event-stream');

var lineNr = 0;

var s = fs.createReadStream('very-large-file.csv')
    .pipe(es.split())
    .pipe(es.mapSync(function(line){

        // pause the readstream
        s.pause();

        lineNr += 1;

        // process line here and call s.resume() when rdy
        // function below was for logging memory usage
        logMemoryUsage(lineNr);

        // resume the readstream, possibly from a callback
        s.resume();
    })
    .on('error', function(err){
        console.log('Error while reading file.', err);
    })
    .on('end', function(){
        console.log('Read entire file.')
    })
);

Please let me know how it goes!