Writing files in Node.js - Mini Project: Build & Apply

Writing Files with Node.js

📖 Scenario: You are building a simple Node.js script to save user notes to a text file on your computer.This is useful for keeping records or logs in many real-world applications.

🎯 Goal: Learn how to write text data to a file using Node.js built-in fs module.You will create a file and write some content into it step-by-step.

📋 What You'll Learn

Use Node.js fs module to write files

Create a string variable with note content

Use fs.writeFileSync to write the content to a file

Name the file note.txt

💡 Why This Matters

🌍 Real World

Writing files is common for saving logs, user data, or configuration in many Node.js applications.

💼 Career

Knowing how to handle file operations is essential for backend developers and automation scripts.

1

Import the fs module

Write a line to import the Node.js fs module using import fs from 'fs';

Node.js

// Your code here

Hint

Use ES module syntax to import fs like this: import fs from 'fs';

2

Create a string variable with note content

Create a variable called noteContent and set it to the string 'This is my first note.'

Node.js

import fs from 'fs';
// Your code here

Hint

Use const to create the variable and assign the exact string.

3

Write the note content to a file

Use fs.writeFileSync to write the content of noteContent to a file named 'note.txt'

Node.js

import fs from 'fs';
const noteContent = 'This is my first note.';
// Your code here

Hint

Call fs.writeFileSync with the filename and the string variable as arguments.

4

Add a confirmation message

Add a line to log 'Note saved successfully!' to the console using console.log

Node.js

import fs from 'fs';
const noteContent = 'This is my first note.';
fs.writeFileSync('note.txt', noteContent);
// Your code here

Hint

Use console.log to show a success message after writing the file.

Practice

1. What is the main purpose of using the fs/promises module in Node.js when writing files?

easy

A. To write files using promises and async/await for cleaner code

B. To read files synchronously

C. To create HTTP servers

D. To manage environment variables

Solution

1. Step 1: Understand the role of fs/promises

   The fs/promises module provides promise-based versions of file system functions, allowing use of async/await.

2. Step 2: Identify the purpose related to writing files

   It is mainly used to write files asynchronously with cleaner syntax compared to callbacks.

3. Final Answer:

   To write files using promises and async/await for cleaner code -> Option A

4. Quick Check:

   fs/promises = async file writing [OK]

Hint: Remember fs/promises is for async file operations [OK]

Common Mistakes:

• Confusing fs/promises with synchronous fs methods
• Thinking fs/promises is for reading environment variables
• Assuming fs/promises creates servers

2. Which of the following is the correct syntax to write 'Hello World' to a file named 'greet.txt' using fs/promises in Node.js?

easy

A. fs.writeFileSync('greet.txt', 'Hello World');

B. fs.promises.write('greet.txt', 'Hello World');

C. await fs.writeFile('greet.txt', 'Hello World');

D. await fs.write('greet.txt', 'Hello World');

Solution

1. Step 1: Recall the correct method name in fs/promises

   The method to write files is writeFile, used with await for promises.

2. Step 2: Check syntax correctness

   await fs.writeFile('greet.txt', 'Hello World'); uses await fs.writeFile('greet.txt', 'Hello World'); which is correct syntax for async write.

3. Final Answer:

   await fs.writeFile('greet.txt', 'Hello World'); -> Option C

4. Quick Check:

   writeFile + await = correct syntax [OK]

Hint: Use await with fs.writeFile for async writing [OK]

Common Mistakes:

• Using writeFileSync without await in async code
• Calling non-existent fs.promises.write method
• Omitting await with promise-based methods

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

import { writeFile } from 'fs/promises';

async function save() {
  await writeFile('data.txt', 'Node.js Rocks!');
  console.log('File saved');
}
save();

medium

A. No output

B. File saved

C. File saved\nNode.js Rocks!

D. SyntaxError

Solution

1. Step 1: Analyze the async function behavior

   The function writes 'Node.js Rocks!' to 'data.txt' asynchronously, then logs 'File saved'.

2. Step 2: Determine console output

   Since writeFile completes before console.log, the output is 'File saved' printed once.

3. Final Answer:

   File saved -> Option B

4. Quick Check:

   Async write then log = 'File saved' [OK]

Hint: Async writeFile logs after completion [OK]

Common Mistakes:

• Expecting file content to print to console
• Confusing syntax error with valid import
• Thinking no output occurs without explicit return

4. Identify the error in the following code that attempts to write to a file:

import fs from 'fs/promises';

async function writeData() {
  fs.writeFile('output.txt', 'Test data');
  console.log('Done');
}
writeData();

medium

A. writeFile method does not exist in fs/promises

B. Incorrect import statement for fs/promises

C. File path 'output.txt' is invalid

D. Missing await before fs.writeFile causing unhandled promise

Solution

1. Step 1: Check async function usage

   The function calls fs.writeFile but does not await it, so the promise is not handled properly.

2. Step 2: Understand consequences of missing await

   Without await, the write operation may not complete before 'Done' logs, and errors won't be caught.

3. Final Answer:

   Missing await before fs.writeFile causing unhandled promise -> Option D

4. Quick Check:

   Always await async fs methods [OK]

Hint: Always await async file writes to avoid errors [OK]

Common Mistakes:

• Forgetting await with async fs methods
• Assuming import syntax is wrong when it is correct
• Blaming file path without checking code first

5. You want to write multiple lines to a file 'log.txt' asynchronously, appending each new line without overwriting. Which approach correctly achieves this using fs/promises?

hard

A. Use await fs.appendFile('log.txt', 'New line\n'); inside an async function

B. Use await fs.writeFile('log.txt', 'New line\n'); repeatedly

C. Use fs.writeFileSync('log.txt', 'New line\n'); inside async function

D. Use fs.appendFileSync('log.txt', 'New line\n'); without async

Solution

1. Step 1: Understand file appending vs overwriting

   To add lines without erasing existing content, appending is needed, not writeFile which overwrites.

2. Step 2: Choose correct async method

   fs.appendFile from fs/promises appends asynchronously and works with await.

3. Final Answer:

   Use await fs.appendFile('log.txt', 'New line\n'); inside an async function -> Option A

4. Quick Check:

   appendFile + await = append lines safely [OK]

Hint: Use appendFile to add lines without erasing [OK]

Common Mistakes:

• Using writeFile which overwrites file content
• Using synchronous methods in async code
• Not adding newline characters when appending