Reading files asynchronously with callbacks in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Node.js asynchronous file read?

Consider the following code snippet that reads a file asynchronously using callbacks. What will be printed to the console?

Node.js

import { readFile } from 'node:fs';

readFile('example.txt', 'utf8', (err, data) => {
  if (err) {
    console.log('Error:', err.message);
  } else {
    console.log('File content:', data);
  }
});

console.log('Read request sent');

A Error: ENOENT: no such file or directory, open 'example.txt'\nRead request sent

B File content: (contents of example.txt)\nRead request sent

C Read request sent\nFile content: (contents of example.txt)

D Read request sent

💡 Hint

Remember that readFile is asynchronous and the callback runs after the main thread continues.

The console.log('Read request sent') runs immediately after calling readFile, before the file is read. The callback runs later and prints the file content or error.

❓ component_behavior

intermediate

What happens if the callback is missing in readFile?

What will happen if you call readFile without providing a callback function?

Node.js

import { readFile } from 'node:fs';

readFile('example.txt', 'utf8');

A A TypeError is thrown because callback is required.

B The program crashes with an unhandled exception.

C Nothing happens; the file is not read.

D The file is read and data is returned synchronously.

💡 Hint

Check the Node.js documentation for readFile function signature.

The readFile function requires a callback to handle the result asynchronously. Omitting it causes a TypeError.

🔧 Debug

advanced

Why does this callback not print the file content?

This code tries to read a file and print its content but only prints 'Read request sent'. What is the issue?

Node.js

import { readFile } from 'node:fs';

readFile('example.txt', 'utf8', (err, data) => {
  if (err) {
    console.error(err);
  }
});

console.log('Read request sent');

A The callback does not print the data when there is no error.

B The file path is incorrect causing silent failure.

C The encoding option 'utf8' is invalid causing no data.

D The console.log outside the callback blocks the callback from running.

💡 Hint

Look at what the callback does when there is no error.

The callback only logs errors but does not print the file content when reading succeeds.

📝 Syntax

advanced

Which option correctly reads a file asynchronously with a callback?

Select the code snippet that correctly reads 'data.txt' asynchronously and logs its content or error.

A readFile('data.txt', (data, err) => { if (err) throw err; console.log(data); });

B readFile('data.txt', (err, data) => { if (err) throw err; console.log(data); });

C readFile('data.txt', 'utf8', (data, err) => { if (err) throw err; console.log(data); });

D readFile('data.txt', 'utf8', (err, data) => { if (err) throw err; console.log(data); });

💡 Hint

Check the order of parameters in the callback and the encoding argument.

The callback parameters are (err, data). Option D correctly passes encoding and uses correct parameter order.

❓ state_output

expert

What is the value of 'result' after this asynchronous readFile call?

Given this code, what will be the value of 'result' after execution?

Node.js

import { readFile } from 'node:fs';

let result = '';

readFile('file.txt', 'utf8', (err, data) => {
  if (!err) {
    result = data;
  }
});

console.log(result);

A Throws a ReferenceError.

B An empty string.

C undefined.

D The content of 'file.txt' as a string.

💡 Hint

Remember that readFile is asynchronous and console.log runs before the callback.

The console.log runs immediately before the callback updates 'result', so it prints the initial empty string.