Reading files with promises (fs.promises) in Node.js

Introduction

Reading files with promises lets you get file content without blocking your program. It helps keep your app fast and smooth.

You want to read a text file and then do something with its content.

You need to read multiple files one after another without freezing your app.

You want to handle errors easily when reading files.

You are building a server that reads configuration files on start.

You want to use modern JavaScript with async/await for cleaner code.

Syntax

Node.js

import { promises as fs } from 'fs';

async function readFileExample() {
  try {
    const data = await fs.readFile('filename.txt', 'utf8');
    console.log(data);
  } catch (error) {
    console.error('Error reading file:', error);
  }
}

Use fs.readFile from fs.promises to get a promise that resolves with file content.

Always use try/catch or .catch() to handle errors when reading files.

Examples

Using .then() and .catch() to handle the promise from reading a file.

Node.js

import { promises as fs } from 'fs';

fs.readFile('example.txt', 'utf8')
  .then(data => console.log(data))
  .catch(err => console.error(err));

Using async/await for cleaner, easier-to-read code.

Node.js

import { promises as fs } from 'fs';

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

read();

Handling errors gracefully when the file does not exist.

Node.js

import { promises as fs } from 'fs';

async function readFile() {
  try {
    const data = await fs.readFile('missing.txt', 'utf8');
    console.log(data);
  } catch (error) {
    console.error('File not found or error:', error.message);
  }
}

readFile();

Sample Program

This program reads the file greeting.txt and prints its content. If the file is missing or unreadable, it shows an error message.

Node.js

import { promises as fs } from 'fs';

async function showFileContent() {
  try {
    const content = await fs.readFile('greeting.txt', 'utf8');
    console.log('File content:');
    console.log(content);
  } catch (error) {
    console.error('Failed to read file:', error.message);
  }
}

showFileContent();

Important Notes

Always specify the encoding like 'utf8' to get a string instead of a buffer.

Promises let you write asynchronous code that looks like normal, step-by-step code.

Use Node.js DevTools or console logs to check if your file path is correct.

Summary

Use fs.promises.readFile to read files asynchronously with promises.

Handle errors with try/catch or .catch() to avoid crashes.

Use async/await for clean and readable asynchronous code.

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