Reading files with promises (fs.promises) in Node.js - Mini Project: Build & Apply

Reading Files with Promises using fs.promises in Node.js

📖 Scenario: You are building a simple Node.js script to read the contents of a text file asynchronously. This is useful when you want your program to keep working without waiting for the file to finish loading.

🎯 Goal: Learn how to use fs.promises to read a file asynchronously and handle the content using promises.

📋 What You'll Learn

Create a variable to import the fs/promises module.

Create a variable with the exact filename example.txt.

Use fs.readFile to read the file content as a string.

Add a .then() method to handle the file content.

Add a .catch() method to handle any errors.

💡 Why This Matters

🌍 Real World

Reading files asynchronously is common in Node.js applications to avoid blocking the program while waiting for file operations. This helps keep apps responsive.

💼 Career

Understanding how to use promises with fs.promises is important for backend developers working with Node.js to handle file input/output efficiently.

1

Import fs.promises and set filename

Create a variable called fs that imports the fs/promises module using import. Then create a variable called filename and set it to the string 'example.txt'.

Node.js

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

Hint

Use import fs from 'fs/promises' to get the promises API of fs. Then assign filename to 'example.txt'.

2

Read the file content with readFile

Use fs.readFile with the variable filename and the option { encoding: 'utf8' } to read the file content as a string. Assign the returned promise to a variable called filePromise.

Node.js

import fs from 'fs/promises';
const filename = 'example.txt';
// Your code here

Hint

Call fs.readFile(filename, { encoding: 'utf8' }) and assign it to filePromise.

3

Handle the file content with then

Add a .then() method to filePromise that takes a parameter called content and returns content. Assign this to a variable called contentPromise.

Node.js

import fs from 'fs/promises';
const filename = 'example.txt';
const filePromise = fs.readFile(filename, { encoding: 'utf8' });
// Your code here

Hint

Use filePromise.then(content => content) and assign it to contentPromise.

4

Add catch to handle errors

Add a .catch() method to contentPromise that takes a parameter called error and returns the string 'Error reading file'. Assign this to a variable called finalPromise.

Node.js

import fs from 'fs/promises';
const filename = 'example.txt';
const filePromise = fs.readFile(filename, { encoding: 'utf8' });
const contentPromise = filePromise.then(content => content);
// Your code here

Hint

Use contentPromise.catch(error => 'Error reading file') and assign it to finalPromise.

Practice

1. What does fs.promises.readFile return when reading a file in Node.js?

easy

A. The file content directly as a string

B. A promise that resolves with the file content

C. A callback function to handle the file content

D. An event emitter for file reading progress

Solution

1. Step 1: Understand fs.promises.readFile behavior

   This method returns a promise that will resolve when the file is read successfully.

2. Step 2: Identify the return type

   Since it returns a promise, you can use await or .then() to get the file content asynchronously.

3. Final Answer:

   A promise that resolves with the file content -> Option B

4. Quick Check:

   fs.promises.readFile returns a promise [OK]

Hint: Remember: fs.promises methods always return promises [OK]

Common Mistakes:

• Thinking it returns file content directly
• Confusing with callback-based fs.readFile
• Expecting an event emitter

2. Which of the following is the correct syntax to read a file using fs.promises.readFile with async/await?

easy

A. const data = await fs.promises.readFile('file.txt');

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

C. fs.promises.readFile('file.txt', (err, data) => {});

D. await fs.readFile('file.txt', 'utf8');

Solution

1. Step 1: Use async/await with promises

   To get the file content, you must await the promise returned by fs.promises.readFile.

2. Step 2: Check syntax correctness

   const data = await fs.promises.readFile('file.txt'); correctly uses await with fs.promises.readFile. const data = fs.promises.readFile('file.txt'); misses await, C uses callback style which is incorrect here, and D uses wrong module method.

3. Final Answer:

   const data = await fs.promises.readFile('file.txt'); -> Option A

4. Quick Check:

   Use await with fs.promises.readFile [OK]

Hint: Always await promises to get their resolved value [OK]

Common Mistakes:

• Omitting await and expecting immediate data
• Using callback style with promises API
• Mixing fs and fs.promises methods

3. What will be logged by this code snippet?

import { promises as fs } from 'fs';

async function read() {
  const content = await fs.readFile('example.txt', 'utf8');
  console.log(typeof content);
}

read();

medium

A. 'undefined'

B. 'object'

C. 'string'

D. Throws an error

Solution

1. Step 1: Understand readFile with encoding

   When you pass 'utf8' as the second argument, the promise resolves with a string containing the file content.

2. Step 2: Check the logged type

   The typeof operator on a string returns 'string', so the console logs 'string'.

3. Final Answer:

   'string' -> Option C

4. Quick Check:

   readFile with 'utf8' returns string [OK]

Hint: Add 'utf8' to get string, else Buffer is returned [OK]

Common Mistakes:

• Forgetting encoding returns Buffer, not string
• Expecting 'object' type for file content
• Not awaiting the promise before logging

4. Identify the error in this code snippet:

import { promises as fs } from 'fs';

async function readFile() {
  const data = fs.readFile('data.txt', 'utf8');
  console.log(data);
}

readFile();

medium

A. Missing await before fs.readFile call

B. Wrong import syntax for fs.promises

C. readFile does not exist in fs.promises

D. Encoding 'utf8' is invalid here

Solution

1. Step 1: Check asynchronous call handling

   The fs.readFile returns a promise, so to get the file content, you must await it.

2. Step 2: Identify the missing await

   Without await, data is a promise object, so logging it shows a promise, not file content.

3. Final Answer:

   Missing await before fs.readFile call -> Option A

4. Quick Check:

   Always await promises to get resolved value [OK]

Hint: Await promises before using their results [OK]

Common Mistakes:

• Logging promise instead of awaited result
• Confusing import syntax for fs.promises
• Passing wrong encoding string

5. You want to read multiple files ['a.txt', 'b.txt', 'c.txt'] concurrently using fs.promises.readFile and get their contents as strings. Which code snippet correctly does this?

hard

A. const contents = await fs.readFile(files, 'utf8');

B. const contents = files.map(f => await fs.readFile(f, 'utf8'));

C. const contents = files.forEach(async f => await fs.readFile(f, 'utf8'));

D. const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8')));

Solution

1. Step 1: Understand concurrent reading with Promise.all

   To read multiple files concurrently, map each filename to a promise and use Promise.all to await all results.

2. Step 2: Analyze each option

   const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8'))); correctly maps files to promises and awaits them all. const contents = files.map(f => await fs.readFile(f, 'utf8')); uses await inside map callback which is invalid syntax. const contents = files.forEach(async f => await fs.readFile(f, 'utf8')); uses forEach which returns undefined. const contents = await fs.readFile(files, 'utf8'); tries to read multiple files at once, which is invalid.

3. Final Answer:

   const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8'))); -> Option D

4. Quick Check:

   Use Promise.all with map for concurrent reads [OK]

Hint: Use Promise.all with map to await multiple promises [OK]

Common Mistakes:

• Using await inside map callback directly
• Using forEach which returns undefined
• Trying to read multiple files in one readFile call