Appending to files in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Appending text to a file with fs.appendFile

What will be the content of the file 'log.txt' after running this code if the file initially contains 'Hello'? Assume no errors occur.

Node.js

import { appendFileSync, readFileSync } from 'node:fs';

appendFileSync('log.txt', ' World');
const content = readFileSync('log.txt', 'utf8');
console.log(content);

A "Hello World"

B " World"

C "Hello"

D "Hello\n World"

💡 Hint

Appending adds new content to the end of the existing file content.

The appendFileSync method adds the string ' World' to the end of the existing file content 'Hello', resulting in 'Hello World'.

❓ Predict Output

intermediate

Asynchronous file append behavior

What will be logged to the console after running this code snippet?

Node.js

import { appendFile } from 'node:fs';

appendFile('notes.txt', 'New note\n', (err) => {
  if (err) throw err;
  console.log('Note added');
});
console.log('Appending started');

A Appending started\nNote added

B Note added\nAppending started

C Appending started

D Note added

💡 Hint

Asynchronous functions run their callback after the main thread continues.

The console.log('Appending started') runs immediately. The callback logs 'Note added' after the append operation finishes asynchronously.

🔧 Debug

advanced

Identifying error in appending to a file

This code is intended to append 'Data' to 'file.txt'. What error will it raise when run?

Node.js

import fs from 'node:fs';

fs.appendFile('file.txt', 'Data', (error) => {
  if (error) throw error;
});

fs.appendFileSync('file.txt', 'More data')

A Error: appendFileSync called without encoding

B Error: Callback must be a function

C TypeError: fs.appendFile is not a function

D No error, both append operations succeed

💡 Hint

Check the usage of appendFile and appendFileSync methods.

Both appendFile and appendFileSync are valid methods. The code correctly uses a callback for appendFile and a synchronous call for appendFileSync. No error occurs.

❓ component_behavior

advanced

Effect of encoding on appending files

What will be the content of 'data.txt' after running this code if the file initially contains 'abc'?

Node.js

import { appendFileSync, readFileSync } from 'node:fs';

appendFileSync('data.txt', Buffer.from([0x20, 0x64, 0x65, 0x66]));
const content = readFileSync('data.txt', 'utf8');
console.log(content);

A "abcdef"

B "abc\u0000def"

C "abc def"

D "abc def"

💡 Hint

Buffer bytes correspond to ASCII characters; 0x20 is a space.

The buffer bytes [0x20, 0x64, 0x65, 0x66] correspond to ' def'. Appending this to 'abc' results in 'abc def'.

📝 Syntax

expert

Correct syntax for appending JSON data to a file

Which option correctly appends a JSON string to 'output.json' without overwriting existing content?

A

import { appendFile } from 'node:fs';
appendFile('output.json', JSON.stringify({a:1}), () => {});

B

import { appendFileSync } from 'node:fs';
appendFileSync('output.json', JSON.stringify({a:1}) + '\n');

C

import { writeFileSync } from 'node:fs';
writeFileSync('output.json', JSON.stringify({a:1}));

D

import { appendFileSync } from 'node:fs';
appendFileSync('output.json', JSON.stringify({a:1}));
appendFileSync('output.json', '\n');

💡 Hint

Appending JSON data as a string with a newline keeps entries separated.

Option B appends the JSON string plus a newline in one call, preserving existing content and formatting. Option B overwrites. Option B appends JSON and newline separately, which works but is less clean. Option B is asynchronous but lacks error handling and newline.