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.

Answer 1

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:

const fs = require('fs'); const readline = require('readline');  async function processLineByLine() {   const fileStream = fs.createReadStream('input.txt');    const rl = readline.createInterface({     input: fileStream,     crlfDelay: Infinity   });   // Note: we use the crlfDelay option to recognize all instances of CR LF   // ('\r\n') in input.txt as a single line break.    for await (const line of rl) {     // Each line in input.txt will be successively available here as `line`.     console.log(`Line from file: ${line}`);   } }  processLineByLine(); 

Or alternatively:

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.

Answer 2

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); });