Reading files with promises (fs.promises) in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the promises API from the fs module.

Node.js

import { [1] } from 'fs';

Drag options to blanks, or click blank then click option'

Apromises

BreadFile

CwriteFile

DcreateReadStream

Attempts:

3 left

💡 Hint

Common Mistakes

Importing 'readFile' directly instead of from 'fs.promises'.

Using 'createReadStream' which is not promise-based.

The promises object from the fs module provides promise-based file system methods.

2fill in blank

medium

Complete the code to read a file named 'data.txt' using promises.

Node.js

const data = await promises.[1]('data.txt', 'utf-8');

Drag options to blanks, or click blank then click option'

AwriteFile

BappendFile

CreadFile

Dunlink

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'writeFile' which writes data instead of reading.

Using 'appendFile' which adds data to a file.

The readFile method reads the contents of a file and returns a promise.

3fill in blank

hard

Fix the error in the async function to correctly read 'info.txt'.

Node.js

async function readInfo() {
  const content = await promises.[1]('info.txt', 'utf-8');
  return content;
}

Drag options to blanks, or click blank then click option'

AreadStream

BreadFileSync

Cread

DreadFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readFileSync' inside an async function with await causes errors.

Using non-existent methods like 'readStream'.

readFile is the correct async method returning a promise. readFileSync is synchronous and not used with await.

4fill in blank

hard

Fill both blanks to handle errors when reading 'notes.txt' with promises.

Node.js

try {
  const notes = await promises.[1]('notes.txt', 'utf-8');
  console.log(notes);
} catch ([2]) {
  console.error('Error reading file:', err);
}

Drag options to blanks, or click blank then click option'

AreadFile

BwriteFile

Cerror

Derr

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'writeFile' instead of 'readFile' to read files.

Not naming the error parameter in catch block.

Use readFile to read the file and catch the error with a variable like err to handle exceptions.

5fill in blank

hard

Fill all three blanks to create an async function that reads 'log.txt' and returns its content or an error message.

Node.js

async function readLog() {
  try {
    const content = await promises.[1]('log.txt', [2]);
    return content;
  } catch ([3]) {
    return 'Failed to read file';
  }
}

Drag options to blanks, or click blank then click option'

AreadFile

B'utf-8'

Cerror

DwriteFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'writeFile' instead of 'readFile'.

Omitting the encoding string or using wrong quotes.

Not naming the error parameter in catch.

The function uses readFile with encoding 'utf-8' and catches errors with the variable error.

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