How to read from stdin line by line in Node

Question

I\'m looking to process a text file with node using a command line call like:

node app.js < input.txt

Each line of the file needs to be proce

Answer 1

In my case the program (elinks) returned lines that looked empty, but in fact had special terminal characters, color control codes and backspace, so grep options presented in other answers did not work for me. So I wrote this small script in Node.js. I called the file tight, but that's just a random name.

#!/usr/bin/env node

function visible(a) {
    var R  =  ''
    for (var i = 0; i < a.length; i++) {
        if (a[i] == '\b') {  R -= 1; continue; }  
        if (a[i] == '\u001b') {
            while (a[i] != 'm' && i < a.length) i++
            if (a[i] == undefined) break
        }
        else R += a[i]
    }
    return  R
}

function empty(a) {
    a = visible(a)
    for (var i = 0; i < a.length; i++) {
        if (a[i] != ' ') return false
    }
    return  true
}

var readline = require('readline')
var rl = readline.createInterface({ input: process.stdin, output: process.stdout, terminal: false })

rl.on('line', function(line) {
    if (!empty(line)) console.log(line) 
})

Answer 2

You can use the readline module to read from stdin line by line:

var readline = require('readline');
var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout,
  terminal: false
});

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

Answer 3

#!/usr/bin/env node

const EventEmitter = require('events');

function stdinLineByLine() {
  const stdin = new EventEmitter();
  let buff = "";

  process.stdin
    .on('data', data => {
      buff += data;
      lines = buff.split(/[\r\n|\n]/);
      buff = lines.pop();
      lines.forEach(line => stdin.emit('line', line));
    })
    .on('end', () => {
      if (buff.length > 0) stdin.emit('line', buff);
    });

  return stdin;
}

const stdin = stdinLineByLine();
stdin.on('line', console.log);

Answer 4

readline is specifically designed to work with terminal (that is process.stdin.isTTY === true). There are a lot of modules which provide split functionality for generic streams, like split. It makes things super-easy:

process.stdin.pipe(require('split')()).on('data', processLine)

function processLine (line) {
  console.log(line + '!')
}

Answer 5

if you want to ask the user number of lines first:

    //array to save line by line 
    let xInputs = [];

    const getInput = async (resolve)=>{
            const readline = require('readline').createInterface({
                input: process.stdin,
                output: process.stdout,
            });
            readline.on('line',(line)=>{
            readline.close();
            xInputs.push(line);
            resolve(line);
            })
    }

    const getMultiInput = (numberOfInputLines,callback)=>{
        let i = 0;
        let p = Promise.resolve(); 
        for (; i < numberOfInputLines; i++) {
            p = p.then(_ => new Promise(resolve => getInput(resolve)));
        }
        p.then(()=>{
            callback();
        });
    }

    //get number of lines 
    const readline = require('readline').createInterface({
        input: process.stdin,
        output: process.stdout,
        terminal: false
    });
    readline.on('line',(line)=>{
        getMultiInput(line,()=>{
           //get here the inputs from xinputs array 
        });
        readline.close();
    })

Answer 6

process.stdin.pipe(process.stdout);