node.js - Read Nth line of file in NodeJS

Question

I'm attempting to extract a single line of a file, given that I know the pathname and the line number, ideally I'd like to do this without reading any more of the file than is necessary.

For the purpose I'm using here, it doesn't matter if this is async or sync.

My current (bad) implementation looks like this:

function get_line(filename, line_no, callback) {
    line_no = parseInt(line_no);
    var data = fs.readFileSync(filename, 'utf8');
    var lines = data.split("
");
    for (var l in lines) {
        if (l == line_no - 1) {
            callback(null, lines[l].trim());
            return;
        }
    }
    throw new Error('File end reached without finding line');
}

I tried to do something with a createReadStream, but the data events never seemed to fire. Can anyone provide a direct solution to this problem, or point me towards some NodeJS filesystem interaction documentation that is a little more example driven than the standard library API docs?

Answer 1

With readable stream

var fs = require('fs');

function get_line(filename, line_no, callback) {
    var stream = fs.createReadStream(filename, {
      flags: 'r',
      encoding: 'utf-8',
      fd: null,
      mode: 0666,
      bufferSize: 64 * 1024
    });

    var fileData = '';
    stream.on('data', function(data){
      fileData += data;

      // The next lines should be improved
      var lines = fileData.split("
");

      if(lines.length >= +line_no){
        stream.destroy();
        callback(null, lines[+line_no]);
      }
    });

    stream.on('error', function(){
      callback('Error', null);
    });

    stream.on('end', function(){
      callback('File end reached without finding line', null);
    });

}

get_line('./file.txt', 1, function(err, line){
  console.log('The line: ' + line);
})

Direct solution:

You should use the slice method instead of a loop.

var fs = require('fs');

function get_line(filename, line_no, callback) {
    var data = fs.readFileSync(filename, 'utf8');
    var lines = data.split("
");

    if(+line_no > lines.length){
      throw new Error('File end reached without finding line');
    }

    callback(null, lines[+line_no]);
}

get_line('./file.txt', 9, function(err, line){
  console.log('The line: ' + line);
})

for (var l in lines) isn't the most efficient way for looping over an array, you should do this instead:

for(var i = 0, iMax = lines.length; i < iMax; i++){/* lines[i] */ }

The asynchronous way:

var fs = require('fs');

function get_line(filename, line_no, callback) {
    fs.readFile(filename, function (err, data) {
      if (err) throw err;

      // Data is a buffer that we need to convert to a string
      // Improvement: loop over the buffer and stop when the line is reached
      var lines = data.toString('utf-8').split("
");

      if(+line_no > lines.length){
        return callback('File end reached without finding line', null);
      }

      callback(null, lines[+line_no]);
    });
}

get_line('./file.txt', 9, function(err, line){
  console.log('The line: ' + line);
})