I'm trying to use writeFile() inside call back function of readFile() from fileSystem module..
I'm a beginner in node.js I have been watching Youtube tutorials
const fs = require('fs');
fs.readFile('readMe.txt','utf8', function(err, data){
fs.writeFile('writeMe.txt', data);
});
console.log('Fire..');
I get an error pasted below.. I don't understand the type of this error.. can someone help me with this?
Fire..!
fs.js:128
throw new ERR_INVALID_CALLBACK();
^
TypeError [ERR_INVALID_CALLBACK]: Callback must be a function
at maybeCallback (fs.js:128:9)
at Object.writeFile (fs.js:1163:14)
at C:\Users\shahzaib laptops\Desktop\NodeJS\pathModule\fileSystem.js:6:6
at FSReqWrap.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:53:3)
You can use the Following snippet to read and write file what need. You this in common and use this if u need in many place. u need to pass the filename to read and write to this method.
Else directly use the callback alone the parameter in method and jsut give the file name static as u have above.
public ReadandWriteFile(fileNameToRead, fileNameToWrite, callback) {
fs.readFile(fileNameToRead, "utf-8", (err, data) => {
if (err) { console.log(err) }
if (data) {
fs.writeFile(fileNameToWrite, data, (err) => {
if (err) { console.log(err) };
if (data) {
callback({ message: 'Successfully Written to File.' })
}
});
}
})
}
Add proper err handling and callback Fn. in writefile
const fs = require('fs');
fs.readFile('readMe.txt','utf8', function(err, data){
if(err) callback or return err
else{
fs.writeFile('writeMe.txt', data,'utf8', function(err2, data2){
if(err2) callback or return err2
else console.log(data2)
});
}
});
console.log('Fire..');
You can use promise for readFile and writeFile.
const fsPromises = require("fs").promises;
async function readAndWrite() {
let filehandle;
try {
filehandle = await fsPromises.readFile("readMe.txt");
console.log(filehandle);
let writeComleted = await fsPromises.writeFile("writeMe.txt", filehandle);
} catch {
throw new Error("something bad happened");
}
}
readAndWrite();
I have written below code in one file:
models/exported.js
module.exports = {
processedList: function(store_name) {
var t;
var tradeIds = exported.find({storename: store_name}, function (err, value) {
if (err) return console.error(err);
return value;
}).select('tid -_id');
}, // Export connection here
};
I have another file in routes
routes/exported.js
var exported = require('../models/exported.js');
var tradeIds = exported.processedList(storename);
console.log('simer'+tradeIds);
}
but I get undefined in console.log. If instead of return statement in processedlist I write console.log then the result gets console. But my requirement is to return data from model file to route file.
I am new to express and node js.
I guidance would be highly appreciated.
Acoording to your question, you want calling a function from route and get return response from your function to route. simple use callback functions.
models/exported.js
module.exports = {
processedList: function (store_name, callback) {
var t;
var tradeIds = exported.find({storename: store_name}, function (err, value) {
if (err) {
callback("error", err)
} else {
callback("success", value)
}
}).select('tid -_id');
}
}
routes/exported.js
var exported = require('../models/exported.js');
exported.processedList('storename', function (err, results) {
if (err == 'error') {
console.log(err);
} else {
console.log(results);
}
});
You are trying sync operation in async environment. processedList may or may not have completed when you try to console log tradeIds. NodeJS would not wait for it to complete because it is asynchronous in nature (by design and it is not a bug). You can pass callback rather than executing this way.
models/exported.js
module.exports = {
processedList: function(store_name, cb) {
var t;
var tradeIds = exported.find({storename: store_name}, function (err, value) {
if (err) return cb(err);
cb(null, value);
}).select('tid -_id');
}, // Export connection here
};
routes/exported.js
var exported = require('../models/exported.js');
exported.processedList(storename, function(err, results) {
if (err) { console.log(err); }
console.log(results);
});
This makes sure that console.log happens only when processedList finishes execution.
I would like to read multiple files asynchronously in NodeJS. It is good to read multiple files at the same time, when the order of reading doesn't matter.
However, I'm trying to write contents of these files together into one file. I can write a file just fine, but how do I make sure that all the files have been read before I write all the contents into that one file?
Using async:
'use strict';
let fs = require('fs'),
async = require('async'),
inputs = ['in1', 'in2'],
output = 'out';
function fuse(inputs, output, callback) {
async.map(inputs, (path, callback) => {
fs.readFile(path, callback);
}, (err, contents) => {
if(error) {
callback(error);
} else {
fs.writeFile(output, contents.reduce((a, b) => {
return a + b;
}), callback);
}
});
}
fuse(inputs, output, (error) => {
if(error) {
console.log('Error: ' + error);
} else {
console.log('OK');
}
});
EDIT:
Using promises:
'use strict';
const fs = require('fs'),
inputs = ['in1', 'in2'],
output = 'out'
// Promisify fs.readFile
function read(file) {
return new Promise((resolve, reject) => {
fs.readFile(file, (error, data) => {
if(error) {
reject(error);
} else {
resolve(data);
}
});
});
}
// Promisify fs.writeFile
function write(file, data) {
return new Promise((resolve, reject) => {
fs.writeFile(file, data, (error) => {
if(error) {
reject(error);
} else {
resolve();
}
});
});
}
Promise.all(inputs.map(read)) // Read all files
.then((data) => { // data will be a array of the data in the files
const outData = data.reduce((a, b) => {
return a + b; // concatenate the data
})
return write(output, outData); // write the output
})
.then(() => {
console.log('OK');
})
.catch((error) => {
console.error(error);
});
(Untested, but general idea's here)
As pointed out by libik, fs-promise, util.promisify or bluebird are alternatives to promisify fs.readFile and fs.writeFile.
User Promises by one of the following method
Create promises, each one is resolved when file is read
Use bluebird to create Promise-like methods for fs
Use fs-promise module
Then save all this promises into array and use Promise.all
Other way around can be iterating variable i.e. var filesRead = 0. When file is read, increase this number filesRead++. After this, always check, if you read all the files, if so, you can do the writing
if (filesRead === numberOfFilesToRead){
//write things
}
How do I check for the existence of a file?
Consider opening or reading the file directly, to avoid race conditions:
const fs = require('fs');
fs.open('foo.txt', 'r', (err, fd) => {
// ...
});
fs.readFile('foo.txt', (err, data) => {
if (!err && data) {
// ...
}
})
Using fs.existsSync:
if (fs.existsSync('foo.txt')) {
// ...
}
Using fs.stat:
fs.stat('foo.txt', function(err, stat) {
if (err == null) {
console.log('File exists');
} else if (err.code === 'ENOENT') {
// file does not exist
fs.writeFile('log.txt', 'Some log\n');
} else {
console.log('Some other error: ', err.code);
}
});
Deprecated:
fs.exists is deprecated.
Using path.exists:
const path = require('path');
path.exists('foo.txt', function(exists) {
if (exists) {
// ...
}
});
Using path.existsSync:
if (path.existsSync('foo.txt')) {
// ...
}
Edit:
Since node v10.0.0we could use fs.promises.access(...)
Example async code that checks if file exists:
function checkFileExists(file) {
return fs.promises.access(file, fs.constants.F_OK)
.then(() => true)
.catch(() => false)
}
An alternative for stat might be using the new fs.access(...):
minified short promise function for checking:
s => new Promise(r=>fs.access(s, fs.constants.F_OK, e => r(!e)))
Sample usage:
let checkFileExists = s => new Promise(r=>fs.access(s, fs.constants.F_OK, e => r(!e)))
checkFileExists("Some File Location")
.then(bool => console.log(´file exists: ${bool}´))
expanded Promise way:
// returns a promise which resolves true if file exists:
function checkFileExists(filepath){
return new Promise((resolve, reject) => {
fs.access(filepath, fs.constants.F_OK, error => {
resolve(!error);
});
});
}
or if you wanna do it synchronously:
function checkFileExistsSync(filepath){
let flag = true;
try{
fs.accessSync(filepath, fs.constants.F_OK);
}catch(e){
flag = false;
}
return flag;
}
A easier way to do this synchronously.
if (fs.existsSync('/etc/file')) {
console.log('Found file');
}
The API doc says how existsSync work:
Test whether or not the given path exists by checking with the file system.
Modern async/await way ( Node 12.8.x )
const fileExists = async path => !!(await fs.promises.stat(path).catch(e => false));
const main = async () => {
console.log(await fileExists('/path/myfile.txt'));
}
main();
We need to use fs.stat() or fs.access() because fs.exists(path, callback) now is deprecated
Another good way is fs-extra
fs.exists(path, callback) and fs.existsSync(path) are deprecated now, see https://nodejs.org/api/fs.html#fs_fs_exists_path_callback and https://nodejs.org/api/fs.html#fs_fs_existssync_path.
To test the existence of a file synchronously one can use ie. fs.statSync(path). An fs.Stats object will be returned if the file exists, see https://nodejs.org/api/fs.html#fs_class_fs_stats, otherwise an error is thrown which will be catched by the try / catch statement.
var fs = require('fs'),
path = '/path/to/my/file',
stats;
try {
stats = fs.statSync(path);
console.log("File exists.");
}
catch (e) {
console.log("File does not exist.");
}
Aug 2021
After reading all posts:
let filePath = "./directory1/file1.txt";
if (fs.existsSync(filePath)) {
console.log("The file exists");
} else {
console.log("The file does not exist");
}
Old Version before V6:
here's the documentation
const fs = require('fs');
fs.exists('/etc/passwd', (exists) => {
console.log(exists ? 'it\'s there' : 'no passwd!');
});
// or Sync
if (fs.existsSync('/etc/passwd')) {
console.log('it\'s there');
}
UPDATE
New versions from V6: documentation for fs.stat
fs.stat('/etc/passwd', function(err, stat) {
if(err == null) {
//Exist
} else if(err.code == 'ENOENT') {
// NO exist
}
});
There are a lot of inaccurate comments about fs.existsSync() being deprecated; it is not.
https://nodejs.org/api/fs.html#fs_fs_existssync_path
Note that fs.exists() is deprecated, but fs.existsSync() is not.
#Fox: great answer!
Here's a bit of an extension with some more options. It's what I've been using lately as a go-to solution:
var fs = require('fs');
fs.lstat( targetPath, function (err, inodeStatus) {
if (err) {
// file does not exist-
if (err.code === 'ENOENT' ) {
console.log('No file or directory at',targetPath);
return;
}
// miscellaneous error (e.g. permissions)
console.error(err);
return;
}
// Check if this is a file or directory
var isDirectory = inodeStatus.isDirectory();
// Get file size
//
// NOTE: this won't work recursively for directories-- see:
// http://stackoverflow.com/a/7550430/486547
//
var sizeInBytes = inodeStatus.size;
console.log(
(isDirectory ? 'Folder' : 'File'),
'at',targetPath,
'is',sizeInBytes,'bytes.'
);
}
P.S. check out fs-extra if you aren't already using it-- it's pretty sweet.
https://github.com/jprichardson/node-fs-extra)
fs.exists has been deprecated since 1.0.0. You can use fs.stat instead of that.
var fs = require('fs');
fs.stat(path, (err, stats) => {
if ( !stats.isFile(filename) ) { // do this
}
else { // do this
}});
Here is the link for the documentation
fs.stats
A concise solution in async await style:
import { stat } from 'fs/promises';
const exists = await stat('foo.txt')
.then(() => true)
.catch(() => false);
async/await version using util.promisify as of Node 8:
const fs = require('fs');
const { promisify } = require('util');
const stat = promisify(fs.stat);
describe('async stat', () => {
it('should not throw if file does exist', async () => {
try {
const stats = await stat(path.join('path', 'to', 'existingfile.txt'));
assert.notEqual(stats, null);
} catch (err) {
// shouldn't happen
}
});
});
describe('async stat', () => {
it('should throw if file does not exist', async () => {
try {
const stats = await stat(path.join('path', 'to', 'not', 'existingfile.txt'));
} catch (err) {
assert.notEqual(err, null);
}
});
});
fs.statSync(path, function(err, stat){
if(err == null) {
console.log('File exists');
//code when all ok
}else if (err.code == "ENOENT") {
//file doesn't exist
console.log('not file');
}
else {
console.log('Some other error: ', err.code);
}
});
After a bit of experimentation, I found the following example using fs.stat to be a good way to asynchronously check whether a file exists. It also checks that your "file" is "really-is-a-file" (and not a directory).
This method uses Promises, assuming that you are working with an asynchronous codebase:
const fileExists = path => {
return new Promise((resolve, reject) => {
try {
fs.stat(path, (error, file) => {
if (!error && file.isFile()) {
return resolve(true);
}
if (error && error.code === 'ENOENT') {
return resolve(false);
}
});
} catch (err) {
reject(err);
}
});
};
If the file does not exist, the promise still resolves, albeit false. If the file does exist, and it is a directory, then is resolves true. Any errors attempting to read the file will reject the promise the error itself.
Well I did it this way, as seen on https://nodejs.org/api/fs.html#fs_fs_access_path_mode_callback
fs.access('./settings', fs.constants.F_OK | fs.constants.R_OK | fs.constants.W_OK, function(err){
console.log(err ? 'no access or dir doesnt exist' : 'R/W ok');
if(err && err.code === 'ENOENT'){
fs.mkdir('settings');
}
});
Is there any problem with this?
For asynchronous version! And with the promise version! Here the clean simple way!
try {
await fsPromise.stat(filePath);
/**
* File exists!
*/
// do something
} catch (err) {
if (err.code = 'ENOENT') {
/**
* File not found
*/
} else {
// Another error!
}
}
A more practical snippet from my code to illustrate better:
try {
const filePath = path.join(FILES_DIR, fileName);
await fsPromise.stat(filePath);
/**
* File exists!
*/
const readStream = fs.createReadStream(
filePath,
{
autoClose: true,
start: 0
}
);
return {
success: true,
readStream
};
} catch (err) {
/**
* Mapped file doesn't exists
*/
if (err.code = 'ENOENT') {
return {
err: {
msg: 'Mapped file doesn\'t exists',
code: EErrorCode.MappedFileNotFound
}
};
} else {
return {
err: {
msg: 'Mapped file failed to load! File system error',
code: EErrorCode.MappedFileFileSystemError
}
};
}
}
The example above is just for demonstration! I could have used the error event of the read stream! To catch any errors! And skip the two calls!
Using typescript and fs/promises in node14
import * as fsp from 'fs/promises';
try{
const = await fsp.readFile(fullFileName)
...
} catch(e) { ...}
It is better to use fsp.readFile than fsp.stator fsp.access for two reasons:
The least important reason - it is one less access.
It is possible that fsp.statand fsp.readFile would give different answers. Either due to subtle differences in the questions they ask, or because the files status changed between the calls. So the coder would have to code for two conditional branches instead of one, and the user might see more behaviors.
in old days before sit down I always check if chair is there then I sit else I have an alternative plan like sit on a coach. Now node.js site suggest just go (no needs to check) and the answer looks like this:
fs.readFile( '/foo.txt', function( err, data )
{
if(err)
{
if( err.code === 'ENOENT' )
{
console.log( 'File Doesn\'t Exist' );
return;
}
if( err.code === 'EACCES' )
{
console.log( 'No Permission' );
return;
}
console.log( 'Unknown Error' );
return;
}
console.log( data );
} );
code taken from http://fredkschott.com/post/2014/03/understanding-error-first-callbacks-in-node-js/ from March 2014, and slightly modified to fit computer. It checks for permission as well - remove permission for to test chmod a-r foo.txt
vannilla Nodejs callback
function fileExists(path, cb){
return fs.access(path, fs.constants.F_OK,(er, result)=> cb(!err && result)) //F_OK checks if file is visible, is default does no need to be specified.
}
the docs say you should use access() as a replacement for deprecated exists()
Nodejs with build in promise (node 7+)
function fileExists(path, cb){
return new Promise((accept,deny) =>
fs.access(path, fs.constants.F_OK,(er, result)=> cb(!err && result))
);
}
Popular javascript framework
fs-extra
var fs = require('fs-extra')
await fs.pathExists(filepath)
As you see much simpler. And the advantage over promisify is that you have complete typings with this package (complete intellisense/typescript)! Most of the cases you will have already included this library because (+-10.000) other libraries depend on it.
You can use fs.stat to check if target is a file or directory and you can use fs.access to check if you can write/read/execute the file. (remember to use path.resolve to get full path for the target)
Documentation:
path.resolve
fs.stat
fs.access
Full example (TypeScript)
import * as fs from 'fs';
import * as path from 'path';
const targetPath = path.resolve(process.argv[2]);
function statExists(checkPath): Promise<fs.Stats> {
return new Promise((resolve) => {
fs.stat(checkPath, (err, result) => {
if (err) {
return resolve(undefined);
}
return resolve(result);
});
});
}
function checkAccess(checkPath: string, mode: number = fs.constants.F_OK): Promise<boolean> {
return new Promise((resolve) => {
fs.access(checkPath, mode, (err) => {
resolve(!err);
});
});
}
(async function () {
const result = await statExists(targetPath);
const accessResult = await checkAccess(targetPath, fs.constants.F_OK);
const readResult = await checkAccess(targetPath, fs.constants.R_OK);
const writeResult = await checkAccess(targetPath, fs.constants.W_OK);
const executeResult = await checkAccess(targetPath, fs.constants.X_OK);
const allAccessResult = await checkAccess(targetPath, fs.constants.F_OK | fs.constants.R_OK | fs.constants.W_OK | fs.constants.X_OK);
if (result) {
console.group('stat');
console.log('isFile: ', result.isFile());
console.log('isDir: ', result.isDirectory());
console.groupEnd();
}
else {
console.log('file/dir does not exist');
}
console.group('access');
console.log('access:', accessResult);
console.log('read access:', readResult);
console.log('write access:', writeResult);
console.log('execute access:', executeResult);
console.log('all (combined) access:', allAccessResult);
console.groupEnd();
process.exit(0);
}());
Using Promise
import { existsSync } from 'fs'
const exists = (filepath) => new Promise((res) => {
existsSync(filepath) ? res(true) : res(false)
})
// Usage #1 (async/await)
const doesItExist = await exists('foo.txt')
if (doesItExist == false) {
// create the file
}
// Usage #2 (thenable)
exists('foo.txt').then(doesItExist => {
if (!doesItExist) {
// create file
}
})
But honestly it's rare to have a case like that,
Usually you'll just go with
import { existsSync as exists } from 'fs'
if (exists('foo.txt')) {
// do something
}