Node.js Write a line into a .txt file

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I want to create a simple Log System, which prints a line before the past line into a txt file by using Node.js, but i dont know how the File System from Node.js works. Can someone explain it ?

Inserting data into the middle of a text file is not a simple task. If possible, you should append it to the end of your file.

The easiest way to append data some text file is to use build-in fs.appendFile(filename, data[, options], callback) function from fs module:

var fs = require('fs')
fs.appendFile('log.txt', 'new data', function (err) {
  if (err) {
    // append failed
  } else {
    // done

But if you want to write data to log file several times, then it'll be best to use fs.createWriteStream(path[, options]) function instead:

var fs = require('fs')
var logger = fs.createWriteStream('log.txt', {
  flags: 'a' // 'a' means appending (old data will be preserved)

logger.write('some data') // append string to your file
logger.write('more data') // again
logger.write('and more') // again

Node will keep appending new data to your file every time you'll call .write, until your application will be closed, or until you'll manually close the stream calling .end:

logger.end() // close string

Node.js read and write file lines, If you're writing a synchronous code, use only the synchronous functions: var fs = require("fs"); fs.readFileSync('./input.txt').toString().split('\n'). Writing to files is a frequent need when programming in any language. Like other programming languages, JavaScript with Node.js makes dealing with the file system intuitive through the use of a module dealing with the operating system's file system. The fs module contains the functionality for manipulating files and dealing with the computing platform's file system.

Simply use fs module and something like this:

fs.appendFile('server.log', 'string to append', function (err) {
   if (err) return console.log(err);

Writing to Files in Node.js, Writing to files is a frequent need when programming in any language. Like other programming languages, JavaScript with Node.js makes dealing with the young and we was dumb but we had heart'; // write to a new file named 2pac.txt fs​. We can also pass an options object or string that specifies the encoding to use,  Run this code by executing node write.js in the terminal and then open up temp.txt in your editor, you should now see the new contents of the file. Creating New Files - The above code will successfully create new files for you should the path to the file not already exist. This is handy as it means you can succinctly create, and write to a new file in one promise.

Step 1

If you have a small file Read all the file data in to memory

Step 2

Convert file data string into Array

Step 3

Search the array to find a location where you want to insert the text

Step 4

Once you have the location insert your text

yourArray.splice(index,0,"new added test");

Step 5

convert your array to string


Step 6

write your file like so


This is not advised if your file is too big

Reading a File Line by Line in Node.js, Dealing with large text files or logs. Sometimes, these files can be absurdly large, with gigabytes or terabytes being stored, and reading through them in entirety is  There are multiple ways of reading a file line by line in Node.js, and the selection of the appropriate approach is entirely a programmer's decision. You should think of the size of the files you plan to process, performance requirements, code style, and modules that are already in the project.

I did a log file which prints data into text file using "Winston" log. The source code is here below,

const { createLogger, format, transports } = require('winston');
var fs = require('fs')
var logger = fs.createWriteStream('Data Log.txt', {`
flags: 'a' 
const os = require('os');
var sleep = require('system-sleep');
var endOfLine = require('os').EOL;
var t = '             ';var s = '         ';var q = '               ';
var array1=[];
var array2=[];
var array3=[];
var array4=[];

array1[0]  =  78;`
array1[1]  =  56;
array1[2]  =  24;
array1[3]  =  34;

for (var n=0;n<4;n++)

for (var k=0;k<4;k++)
array3[k]=Buffer.from('                    ');

for (var a=0;a<4;a++)  

for (m=0;m<4;m++)

logger.write('Time'+(q+'  '))
logger.write('Data 01'+t);
logger.write('Data 02'+t); 
logger.write('Data 03'+t);
logger.write('Data 04'+t)

enter code here`enter code here`

function mydata()      //user defined function
for ( n = 0; n < 4; n++) 

for (;;)
var now = new Date();
var dateFormat = require('dateformat');
var date = dateFormat(now,"isoDate");
var time = dateFormat(now, "h:MM:ss TT ");
var datechar = date.toString();
var timechar = time.toString();

Writing files with Node, The easiest way to write to files in Node.js is to use the fs. writeFileSync('/Users​/flavio/test.txt', content) //file written successfully } catch (err)  By using the fs module we can read and write files in both synchronous and asynchronous way. There are many ways in which we can read and write data to file. Lets have a look on each of them one by one. JavaScript Read and Write to Text File Method 1: Using Node.js. First is by using writefile and readFile method in node.js environment. writeFile:

Node.js Write a line into a .txt file - node.js - html, Node.js Write a line into a .txt file - node.js. The easiest way to append data some text file is to use build-in fs.appendFile(filename, data[, options], callback)  Currently there are three ways to write a file: fs.write(fd, buffer, offset, length, position, callback) You need to wait for the callback to ensure that the buffer is written to disk. It's not buffered. fs.writeFile(filename, data, [encoding], callback) All data must be stored at the same time; you cannot perform sequential writes.

node.js, write to file, node.js, write to file Write to file, will overwrite the existing content. 1. 2. 3. fs.​writeFile( 'out.txt' , "hello, this line written by fs.writeFile function.\n" , function (err){​. Using Node.js v8 or later has a new feature that converts normal function into an async function. util.promisify. It's an awesome feature. Here's the example of parsing 10000 numbers from the txt file into an array, counting inversions using merge sort on the numbers.

Reading and Writing Files With NodeJS, This line does the job of importing the fs package and allowing us to utilize it within our own code. fs.readFile("temp.txt", function(err, buf)  Home » Nodejs » node.js: read a text file into an array. (Each line an item in the array.) (Each line an item in the array.) node.js: read a text file into an array.

  • So, you want to insert a line into the middle of a text file, right? It'll be much easier to append strings to the end.
  • Yes, ok how can i append a string at the end ?
  • Possible duplicate of How to append to a file in Node?
  • It is necessary to save the file ? If i use fs.appendFile, the file is still empty ?
  • appendFile will open text file, append new data to it, and save it afterwards. So, no, you don't need to save it yourself. It your file is empty, try checking err in appendFile callback.
  • Thanks for mentioning both one time and multiple time options.
  • does write put a new line?