Principle and Common Uses of Node. js fs Module

JavaScript has no ability to operate files, but Node can do it. Node provides a module to operate file system, which is a very important and high-frequency module used in Node, and is a module system that must be mastered absolutely.

fs module provides a lot of interfaces, and here we mainly talk about some commonly used interfaces.

1. Quick review of commonly used API

fs. stat Detection is a file or a directory

const fs = require('fs')
fs.stat('hello.js', (error,stats)=>{
  if(error) {
    console.log(error)
  } else {
    console.log(stats)
    console.log(` Documents: ${stats.isFile()}`)
    console.log(` Directory: ${stats.isDirectory()}`)
  }
})

fs. mkdir Create Directory

const fs = require('fs')
fs.mkdir('logs', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Directory created successfully! ')
  }
})

fs. rmdir delete directory

const fs = require('fs')
fs.rmdir('logs', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' The directory was successfully deleted  logs')
  }
})

fs. writeFile Create Write File

const fs = require('fs')
fs.writeFile('logs/hello.log',' How do you do ~\n', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Write to file successfully ');
  }
})

fs. appendFile append file

const fs = require('fs')
fs.appendFile('logs/hello.log','hello~\n', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Write to file successfully ');
  }
})

fs. readFile Read File

const fs = require('fs')
fs.readFile('logs/hello.log','utf-8', (error, data) => {
  if(error) {
    console.log(error)
  } else {
    console.log(data);
  }
})

fs. unlink delete file

const fs = require('fs')
fs.unlink(`logs/${file}`, error => {
  if(error) {
    console.log(error)
  } else {
    console.log(` File deleted successfully:  ${file}`)
  }
})

fs. readdir Read Directory

const fs = require('fs')
fs.readdir('logs', (error, files) => {
  if(error) {
    console.log(error)
  } else {
    console.log(files);
  }
})

fs. rename rename, and you can also change the file storage path

const fs = require('fs')
fs.rename('js/hello.log', 'js/greeting.log', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Rename succeeded ')
  }
})

2. Use of the third party npm package mkdirp

mkdirp can not only create folders, but also create multiple layers of folders, similar to the mkdir-p command

midir -p tmp/foo/bar/baz

The above command can also create multiple layers of folders in the current directory.

The following code generates a multilevel folder in the current directory

const mkdirp = require('mkdirp')
mkdirp('tmp/foo/bar/baz').then(made = > console. log (` Create directory at: ${made} `))
//Create directory at:/Users/zhangbing/github/CodeTest/Node/fs/tmp

3. Practical examples

Actual Combat 1

Determine if there is an upload directory on the server. If not, create this directory, and if so, do not do anything

const fs = require('fs')

const path = './upload'
fs.stat(path, (err, data) => {
  if(err) {
    //  Performing the creation of a directory 
    mkdir(path)
    return
  }
  if(data.isDirectory()) {
    console.log('upload Directory exists ');
  }else{
    //  Delete the file first, and then perform the creation of the directory 
    fs.unlink(path, err => {
      if(!err) {
        mkdir(path)
      }
    })
  }
})

function mkdir(dir) {
  fs.mkdir(dir, err => {
    if(err) {
      console.log(err);
      return
    }
  })
}

Actual Combat 2

There are images css js and index. html under the wwwroot folder. Find all the directories under the wwwroot directory and put them in an array

Using synchronous methods

const fs = require('fs')
fs.mkdir('logs', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Directory created successfully! ')
  }
})

0

Use async/await mode

const fs = require('fs')
fs.mkdir('logs', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Directory created successfully! ')
  }
})

1

4. Pipeline flow

Pipeline provides a mechanism from output stream to input stream. Usually, we use it to get data from one stream and pass it to another stream. In the following example, we read the contents of one file and write the contents to another file.

const fs = require("fs")
//Create 1 readable stream
const readerStream = fs.createReadStream('input.txt')
//Create 1 writable stream
const writerStream = fs.createWriteStream('output.txt')
//Pipeline read and write operations
//Read the contents of the input. txt file and write the contents to the output. txt file
readerStream.pipe(writerStream)
console. log ("Program finished")

fs. createReadStream Reading data from a file stream

const fs = require('fs')
fs.mkdir('logs', error => {
  if(error) {
    console.log(error)
  } else {
    console.log(' Directory created successfully! ')
  }
})

2

fs. createWriteStream write to file

const fs = require("fs")
const data = 'I got the data from the database, I want to save it'
//Create a writable stream to the file output. txt
const writerStream = fs.createWriteStream('output.txt')
//Write data using utf8 encoding
writerStream.write(data,'UTF8')
//Mark end of file
writerStream.end()
//Handle flow events-- > finish event
writerStream.on('finish', () = > {
/*finish-Triggered when all data has been written to the underlying system. */
console. log ("Write complete. ")
})
writerStream.on('error', err = > {
console.log(err.stack);
})
console. log ("Program finished")

Actual combat: copying pictures

There is a picture 2020. png in the project root directory, copy it to/wwwroot/images

The code is as follows

const fs = require("fs")

const readStream = fs.createReadStream('./2020.png')
const writeStream = fs.createWriteStream('./wwwroot/images/2021.png')

readStream.pipe(writeStream)

It is important to note that the directory 1 to be written to fs. createWriteStream must carry the file name to be copied, that is, it cannot be written as fs. createWriteStream ('./wwwroot/images/') otherwise the following error will be reported under macOS:

Error: EISDIR: illegal operation on a directory, open < directory >