Writing files in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Node.js file write operation?

Consider this Node.js code snippet that writes to a file asynchronously. What will be logged to the console?

Node.js

import { writeFile } from 'node:fs/promises';

async function writeData() {
  try {
    await writeFile('test.txt', 'Hello World');
    console.log('Write successful');
  } catch (err) {
    console.log('Write failed');
  }
}

writeData();

A SyntaxError

B Write failed

C No output

D Write successful

💡 Hint

The writeFile function from 'fs/promises' returns a promise that resolves on success.

The code uses the async writeFile function correctly with await inside a try-catch block. If the write succeeds, it logs 'Write successful'.

❓ component_behavior

intermediate

What happens if you try to write to a file without permissions?

Given this Node.js code snippet, what will be the console output if the process lacks write permission to 'readonly.txt'?

Node.js

import { writeFile } from 'node:fs/promises';

async function writeData() {
  try {
    await writeFile('readonly.txt', 'Data');
    console.log('Write successful');
  } catch (err) {
    console.log(err.code);
  }
}

writeData();

A EACCES

B SyntaxError

C ENOENT

D Write successful

💡 Hint

EACCES is the error code for permission denied in Node.js.

If the file is not writable due to permissions, writeFile throws an error with code 'EACCES'. The catch block logs this code.

📝 Syntax

advanced

Which option correctly appends text to a file asynchronously?

Select the code snippet that appends 'More data' to 'log.txt' using Node.js fs/promises module.

A

import { writeFile } from 'node:fs/promises';
await writeFile('log.txt', 'More data', { flag: 'a' });

B

import { appendFile } from 'node:fs/promises';
await appendFile('log.txt', 'More data');

C

import { writeFile } from 'node:fs';
writeFile('log.txt', 'More data', { flag: 'a' });

D

import { appendFileSync } from 'node:fs/promises';
await appendFileSync('log.txt', 'More data');

💡 Hint

Appending can be done by writeFile with flag 'a' or appendFile function.

Option A correctly uses writeFile with flag 'a' to append asynchronously. Option B is also valid because appendFile is an async function that can be awaited inside an async function. Option C uses callback fs module without await. Option D uses appendFileSync incorrectly from promises module.

🔧 Debug

advanced

Why does this code throw an error when writing a file?

Identify the cause of the error in this Node.js code snippet:

Node.js

import fs from 'fs/promises';

fs.writeFile('output.txt', 'Hello').then(() => {
  console.log('Done');
});

fs.writeFile('output.txt', 'World');

A writeFile requires a callback function, missing here.

B fs module import is incorrect, should be 'node:fs/promises'.

C Second writeFile call is missing await or then, causing unhandled promise rejection.

D Cannot call writeFile twice on the same file.

💡 Hint

Promises must be handled to avoid unhandled rejections.

The second writeFile call returns a promise that is not handled with await or then, which can cause unhandled promise rejection errors.

🧠 Conceptual

expert

What is the effect of using the 'wx' flag in writeFile options?

In Node.js fs/promises writeFile, what happens if you use the option { flag: 'wx' } when writing to a file?

A The write operation is performed with exclusive lock preventing other writes.

B The file is created and written only if it does not already exist; otherwise, an error is thrown.

C The file is appended to if it exists; otherwise, it is created.

D The file is overwritten regardless of existing content.

💡 Hint

The 'x' flag means exclusive creation in file system flags.

The 'wx' flag means write and exclusive create. It causes writeFile to fail if the file already exists, preventing overwriting.