how to fix this error TypeError [ERR_INVALID_CALLBACK]: Callback must be a function

I am a beginner to the nodejs. When I type the below, the code error occurs like this:

> TypeError [ERR_INVALID_CALLBACK]: Callback must be a function

var fs = require('fs');
fs.readFile('readMe.txt', 'utf8', function (err, data) {
  fs.writeFile('writeMe.txt', data);
});

Fs.writeFile() according to the documentation here takes ( file, data[, options]and callback ) params so your code will be like this :

 var fs = require('fs');
 fs.readFile('readMe.txt', 'utf8', function (err, data) {
  fs.writeFile('writeMe.txt', data, function(err, result) {
     if(err) console.log('error', err);
   });
 });

fs.writeFile(...) requires a third (or fourth) parameter which is a callback function to be invoked when the operation completes. You should either provide a callback function or use fs.writeFileSync(...)

See node fs docs for more info.

Since node 10, it is mandatory to pass a callback on fs.writefile()

Node.js documented the purpose for the change: https://github.com/nodejs/node/blob/master/doc/api/deprecations.md#dep0013-fs-asynchronous-function-without-callback

You could add an empty callback like this fs.writeFile('writeMe.txt', data, () => {})

you also use like this

var file2 =  fs.readFileSync("./Public/n2.jpeg")

You simply could use the sync function

var fs = require('fs');
fs.readFileSync('readMe.txt', 'utf8', function (err, data) {
  fs.writeFileSync('writeMe.txt', data);
});

or use callback function

This error hit me in the face when I was doing the following;

var hello = myfunction( callme() );

rather than

var hello = myfunction( callme );

var fs = require('fs');

fs.readFile('readme.txt', 'utf8', function(err, data) {
    fs.writeFile('writemeee.txt', data, function(err, result) {

        if (err) console.log('error', err);

    });
});

you can import the fs module from the fs/promises as they are promise-fied version of the modules so we don't need to use the callback function unnecessarily.

import fs from 'fs/promises';

fs.readFileSync('readMe.txt', 'utf8', function (err, data) {
fs.writeFileSync('writeMe.txt', data);`});`

try this .I have written the code using Promises.

const {readFile} = require('fs');
const {writeFileSync} = require('fs');
const readText = (path)=>{
 return new Promise((resolve,reject) => {
    readFile(path,'utf8',(err,result)=>{
      if(err)
        reject(err);
      else
        resolve(result);
      })
  })
}

readText('./contents/sample.txt')
  .then(val=>writeFileSync('./results.txt',val))
  .catch(err=>console.log(err));