Read a file one line at a time in node.js?


Question

I am trying to read a large file one line at a time. I found a question on Quora that dealt with the subject but I'm missing some connections to make the whole thing fit together.

 var Lazy=require("lazy");
 new Lazy(process.stdin)
     .lines
     .forEach(
          function(line) { 
              console.log(line.toString()); 
          }
 );
 process.stdin.resume();

The bit that I'd like to figure out is how I might read one line at a time from a file instead of STDIN as in this sample.

I tried:

 fs.open('./VeryBigFile.csv', 'r', '0666', Process);

 function Process(err, fd) {
    if (err) throw err;
    // DO lazy read 
 }

but it's not working. I know that in a pinch I could fall back to using something like PHP, but I would like to figure this out.

I don't think the other answer would work as the file is much larger than the server I'm running it on has memory for.

1
508
3/7/2016 8:24:15 PM

Accepted Answer

Since Node.js v0.12 and as of Node.js v4.0.0, there is a stable readline core module. Here's the easiest way to read lines from a file, without any external modules:

var lineReader = require('readline').createInterface({
  input: require('fs').createReadStream('file.in')
});

lineReader.on('line', function (line) {
  console.log('Line from file:', line);
});

The last line is read correctly (as of Node v0.12 or later), even if there is no final \n.

UPDATE: this example has been added to Node's API official documentation.

708
8/29/2018 9:37:30 PM

For such a simple operation there shouldn't be any dependency on third-party modules. Go easy.

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

var rd = readline.createInterface({
    input: fs.createReadStream('/path/to/file'),
    output: process.stdout,
    console: false
});

rd.on('line', function(line) {
    console.log(line);
});

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