JavaScript —

Asynchronously read, write and append files
Read a dir recursively

In this tutorial, we’ll learn how to read, write, and append to files in an asynchronous manner. We’re going to write a simple program that grab text from a file, and saves it in a new file. We’ll also learn how to read a dir in an asynchronous manner recursively.

  1. Read file
  2. Write file
  3. Append file
  4. Read dir / folder
  5. Read dir tree

First we’ll require our dependencies:

const fs = require('fs')

The fs is the core modules so there’s no need to install this dependency. See documentation: https://nodejs.org/api/fs.html.

Asynchronous read file

The readFile returns raw buffer if character encoding is not specified.

/*Simple async example*/
const fs = require('fs')
const filePath = 'path/file.txt'

fs.readFile(filePath, (err, data) => {
 if (err) throw err
 console.log(data)
 //<Buffer 61 62 63 0d 0a 64 65 66 0d...
})

If the encoding option is specified then readFile returns a string:

const fs = require('fs')
const filePath = 'path/file.txt'

fs.readFile(filePath, 'utf-8', (err, data) => {
 if (err) throw err
 console.log(data)
 //abc def... 
})

Asynchronous write file

Next, we’ll write the content to a new file and save:

const fs = require('fs')
const filePath = 'path/file.txt'
const newFile = 'path/newfile.txt'

fs.readFile(filePath, (err, data) => {
 if (err) throw err
 fs.writeFile(newFile, data, (err) => {
  if (err) throw err
  console.log(newFile + ' saved')
 })
})

The writeFile writes data to a file, replacing the file if it already exists without giving any warning. data can be a string or a buffer.

Asynchronous append file

Finally, let’s append some text to previous file so we can keep a record:

const fs = require('fs')
const filePath = 'path/file.txt'
const newFile = 'path/newfile.txt'

fs.readFile(filePath, (err, data) => {
 if (err) throw err
 fs.writeFile(newFile, data, (err) => {
  if (err) throw err
  let appendTxt = (new Date) + " Text copied to " + newFile
  fs.appendFile(filePath, appendTxt, (err) => {
   if (err) throw err;
   console.log(filePath + ' some data appended');
  }) 
 })
})

appendFile append data to a file. It creates new file if it if it does not yet exist.

Asynchronously read dir / folder

The Node.js implementation of readdir is a simple command to read a directory.  It is the asynchronous implementation of the file-system directory read:

fs.readdir('./', (err, files) => {
 if(err) throw err
 files.forEach(entry => { 
  console.log (entry)
 })
})

This will print all files and folder from the given path on command terminal.

Asynchronously read dir tree (recursive operation)

 

const fs = require('fs')
const path = require('path')

function readTree (entry) {
 fs.lstat(entry, (err,stat) => {
  if (err) throw err
  if (stat.isDirectory()){
   fs.readdir(entry, (err,files) => {
    if (err) throw err
    files.forEach( file => {
     readTree(path.join(entry,file))
    }) 
   })
  } else {
   console.log (entry)
  }
 })
}

readTree (path/of/dir)

The path.join method is a useful utility that normalizes paths across platforms, since Windows uses back slashes \ whilst others use forward slashes / to denote path segments.

A lstat provides information about the path which store in stat variable.

stat.isDirectory() return true if the given path is a directory.

Next, we'll explore the fs.Stats class and also discuss the difference between the synchronous and asynchronous operations in Node.js.