Nodejs - read line by line from file, perform async action for each line and resume

Question

I'm trying to read a file line by line, perform some action that has a callback and when the function finishes to resume line reading. For example:

var fs = require('fs');
var readline = require('readline');
var stream = require('stream');
var instream = fs.createReadStream('./phrases.txt');
var outstream = new stream;
var rl = readline.createInterface(instream, outstream);
rl.on('line', function (line) {
  rl.pause();
  setTimeout(function () {
    console.log('resuming');
    rl.resume();
  }, 2000);
});

I was under the impression the example above should basically read a line, wait for 2 seconds, console.log and then continue to the next line. What really happens is that it waits for the first 2 seconds and then spews out lots of console.log

Answer 1

Line by Line module helps you reading large text files, line by line, without buffering the files into memory.

You can process the lines asynchronously. This is the example provided:

var LineByLineReader = require('line-by-line'),
    lr = new LineByLineReader('big_file.txt');

lr.on('error', function (err) {
    // 'err' contains error object
});

lr.on('line', function (line) {
    // pause emitting of lines...
    lr.pause();

    // ...do your asynchronous line processing..
    setTimeout(function () {

        // ...and continue emitting lines.
        lr.resume();
    }, 100);
});

lr.on('end', function () {
    // All lines are read, file is closed now.
});