Reading files synchronously in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this synchronous file read code?

Consider the following Node.js code snippet that reads a file synchronously. What will be printed to the console?

Node.js

import fs from 'fs';
const data = fs.readFileSync('example.txt', 'utf8');
console.log(typeof data);

A "undefined"

B "string"

C "Buffer"

D "object"

💡 Hint

The encoding option affects the type of the returned data.

When you specify 'utf8' encoding in readFileSync, the returned data is a string.

Without encoding, it returns a Buffer object.

❓ component_behavior

intermediate

What happens if the file does not exist in synchronous read?

Given this code snippet, what will happen if 'missing.txt' does not exist?

Node.js

import fs from 'fs';
try {
  const content = fs.readFileSync('missing.txt', 'utf8');
  console.log(content);
} catch (err) {
  console.log('Error caught');
}

A It prints an empty string.

B It prints 'undefined'.

C The catch block runs and prints 'Error caught'.

D The program crashes with an uncaught error.

💡 Hint

Synchronous read throws an error if the file is missing.

readFileSync throws an error if the file is not found.

The try...catch block catches this error and prints 'Error caught'.

📝 Syntax

advanced

Which option correctly reads a file synchronously with UTF-8 encoding?

Select the code snippet that correctly reads 'data.txt' synchronously as a UTF-8 string.

A const content = fs.readFileSync('data.txt', 'utf8');

B const content = fs.readFileSync('data.txt', 'utf-8');

C const content = fs.readFileSync('data.txt', {encoding: 'utf8'});

D const content = fs.readFileSync('data.txt', {encoding: 'utf-8'});

💡 Hint

Check the exact spelling and case of encoding options.

Options A and C use the correct encoding 'utf8' which is accepted by Node.js.

Options B and D use 'utf-8' with a dash, which is not recognized and causes an error.

🔧 Debug

advanced

Why does this synchronous read code throw an error?

Identify the cause of the error in this code snippet:

Node.js

import fs from 'fs';
const content = fs.readFileSync('file.txt');
console.log(content.toString('utf8'));

A readFileSync returns a string, so calling toString with encoding causes an error.

B toString does not accept encoding as argument, causing a TypeError.

C Missing encoding in readFileSync causes a syntax error.

D readFileSync returns a Buffer by default, so toString with encoding is valid and no error occurs.

💡 Hint

Check what type readFileSync returns when encoding is not specified.

Without specifying encoding, readFileSync returns a Buffer.

Buffer's toString method accepts an encoding argument, so this code works without error.

🧠 Conceptual

expert

How does synchronous file reading affect Node.js event loop?

Choose the correct statement about synchronous file reading in Node.js.

A Synchronous file reading blocks the event loop, pausing all other operations until complete.

B Synchronous file reading runs in a separate thread, so it does not block the event loop.

C Synchronous file reading queues the read operation and continues immediately without blocking.

D Synchronous file reading only blocks the current callback but allows other events to process.

💡 Hint

Think about what 'synchronous' means for single-threaded Node.js.

Synchronous operations block the main thread in Node.js, stopping all other JavaScript execution until they finish.

This means the event loop is paused during synchronous file reads.

Practice

1. What does reading a file synchronously in Node.js mean?

easy

A. The file is read only if the program is connected to the internet.

B. The program waits until the file is fully read before moving on.

C. The file is read in the background while the program continues.

D. The file is read multiple times at once to speed up reading.

Solution

1. Step 1: Understand synchronous reading

   Reading a file synchronously means the program pauses and waits for the file to be fully read before continuing.

2. Step 2: Compare with asynchronous reading

   Asynchronous reading allows the program to continue running while the file is being read, which is not the case here.

3. Final Answer:

   The program waits until the file is fully read before moving on. -> Option B

4. Quick Check:

   Synchronous reading = wait for file read [OK]

Hint: Synchronous means wait until done before next step [OK]

Common Mistakes:

• Confusing synchronous with asynchronous reading
• Thinking file reads happen in parallel automatically
• Assuming synchronous reading is faster

2. Which of the following is the correct way to read a file synchronously using Node.js's fs module?

easy

A. const data = fs.readSync('file.txt');

B. const data = fs.readFile('file.txt');

C. const data = fs.readFileAsync('file.txt');

D. const data = fs.readFileSync('file.txt', 'utf8');

Solution

1. Step 1: Identify the synchronous read method

   Node.js's fs module provides readFileSync() to read files synchronously.

2. Step 2: Check method usage and parameters

   readFileSync requires the file path and optionally encoding like 'utf8' to get a string.

3. Final Answer:

   const data = fs.readFileSync('file.txt', 'utf8'); -> Option D

4. Quick Check:

   readFileSync is synchronous read method [OK]

Hint: Use readFileSync with encoding for synchronous read [OK]

Common Mistakes:

• Using readFile instead of readFileSync for sync reading
• Missing encoding parameter to get string output
• Using non-existent methods like readFileAsync or readSync

3. What will be the output of this code snippet?

const fs = require('fs');
const content = fs.readFileSync('example.txt', 'utf8');
console.log(typeof content);

medium

A. 'string'

B. 'buffer'

C. 'undefined'

D. 'object'

Solution

1. Step 1: Understand readFileSync output with encoding

   When readFileSync is called with 'utf8' encoding, it returns a string containing the file content.

2. Step 2: Check typeof operator on string

   typeof on a string returns 'string'.

3. Final Answer:

   'string' -> Option A

4. Quick Check:

   readFileSync with 'utf8' returns string [OK]

Hint: readFileSync with 'utf8' returns string type [OK]

Common Mistakes:

• Assuming output is a Buffer without encoding
• Confusing typeof output with file content
• Expecting 'object' or 'buffer' instead of 'string'

4. Identify the error in this code snippet that reads a file synchronously:

const data = fs.readFileSync('data.txt');
console.log(data.toString('utf8'));

medium

A. fs module is not imported correctly.

B. toString() should not have 'utf8' as argument here.

C. Missing encoding in readFileSync causes error.

D. readFileSync requires a callback function.

Solution

1. Step 1: Check the import statement

   The code snippet is missing the line to import the fs module: const fs = require('fs');

2. Step 2: Consequence of missing import

   Without importing fs, fs.readFileSync will throw ReferenceError: fs is not defined.

3. Step 3: Why other options are incorrect

   A: readFileSync is synchronous, no callback needed.
   B: Without encoding, returns Buffer; Buffer.toString('utf8') is valid.
   C: Missing encoding returns Buffer, no error.

4. Final Answer:

   fs module is not imported correctly. -> Option A

5. Quick Check:

   Missing fs import causes ReferenceError [OK]

Hint: Require 'fs' module before using fs methods [OK]

Common Mistakes:

• Thinking readFileSync needs a callback
• Assuming missing encoding causes error
• Believing toString cannot take encoding argument

5. You want to read a small configuration file synchronously and handle errors properly. Which code snippet correctly does this?

hard

A. const fs = require('fs');
const config = fs.readFileSync('config.json');
console.log(config.toString());

B. const fs = require('fs');
const config = fs.readFileSync('config.json', 'utf8', (err, data) => {
if (err) throw err;
console.log(data);
});

C. const fs = require('fs');
try {
const config = fs.readFileSync('config.json', 'utf8');
console.log(config);
} catch (err) {
console.error('Error reading file:', err.message);
}

D. const fs = require('fs');
fs.readFileSync('config.json', 'utf8').then(data => console.log(data));

Solution

1. Step 1: Use try-catch to handle errors in synchronous reading

   Since readFileSync throws errors on failure, wrapping it in try-catch is necessary to handle errors gracefully.

2. Step 2: Check each option for correctness

   const fs = require('fs'); try { const config = fs.readFileSync('config.json', 'utf8'); console.log(config); } catch (err) { console.error('Error reading file:', err.message); } uses try-catch correctly and reads file synchronously with encoding. const fs = require('fs'); const config = fs.readFileSync('config.json', 'utf8', (err, data) => { if (err) throw err; console.log(data); }); incorrectly uses a callback with readFileSync which does not accept callbacks. const fs = require('fs'); const config = fs.readFileSync('config.json'); console.log(config.toString()); reads without encoding and does not handle errors. const fs = require('fs'); fs.readFileSync('config.json', 'utf8').then(data => console.log(data)); incorrectly uses then() on readFileSync which returns data directly, not a promise.

3. Final Answer:

   const fs = require('fs'); try { const config = fs.readFileSync('config.json', 'utf8'); console.log(config); } catch (err) { console.error('Error reading file:', err.message); } -> Option C

4. Quick Check:

   Use try-catch with readFileSync for error handling [OK]

Hint: Wrap readFileSync in try-catch to catch errors [OK]

Common Mistakes:

• Using callbacks with readFileSync
• Not handling errors causing crashes
• Using promises with synchronous methods