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

by

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.

More Related Contents:

  1. Why does Node.js’ fs.readFile() return a buffer instead of string?
  2. Write objects into file with Node.js
  3. What is the most efficient medium for both Node.js and Javascript to read and write a file?
  4. Error: Can’t set headers after they are sent to the client
  5. Why am I getting a ReferenceError: AbortController is not defined in Discord.js v13?
  6. Send message to specific client with socket.io and node.js
  7. How to print a stack trace in Node.js?
  8. ReferenceError: fetch is not defined
  9. for await of VS Promise.all
  10. Difference between “process.stdout.write” and “console.log” in node.js?
  11. How to create a simple http proxy in node.js?
  12. How to sort in mongoose?
  13. Why {} + {} is NaN only on the client side? Why not in Node.js?
  14. How to run untrusted code serverside?
  15. Load and execute external js file in node.js with access to local variables?
  16. Puppeteer log inside page.evaluate
  17. Bind variables to callback function
  18. Gulps gulp.watch not triggered for new or deleted files?
  19. JavaScript classes with getter and setter cause RangeError: Maximum call stack size exceeded
  20. Why couldn’t popular JavaScript runtimes handle synchronous-looking asynchronous script?
  21. stop all instances of node.js server
  22. Wait promise inside for loop
  23. How to run Node.js app with ES6 features enabled?
  24. How does require() in node.js work?
  25. how to save canvas data to file
  26. Node.js – console.log does not show items in array but instead shows [Object]
  27. Websocket API to replace REST API? [closed]
  28. Connecting to remote SSH server (via Node.js/html5 console)
  29. Why I got document is not defined error in puppeteer?
  30. puppeteer wait for page/DOM updates – respond to new items that are added after initial loading

Leave a Comment