Reading files with promises (fs.promises) in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code reading a file?

Consider this Node.js code using fs.promises to read a file named 'example.txt'. What will be logged to the console?

Node.js

import { promises as fs } from 'fs';

async function readFile() {
  try {
    const data = await fs.readFile('example.txt', 'utf8');
    console.log(data.trim());
  } catch (error) {
    console.log('Error:', error.message);
  }
}

readFile();

A SyntaxError due to missing callback in readFile

B The trimmed content of 'example.txt' is printed.

C undefined

D Error: ENOENT: no such file or directory, open 'example.txt'

💡 Hint

Check how fs.promises.readFile returns a promise and how await handles it.

fs.promises.readFile returns a promise that resolves with the file content as a string when 'utf8' encoding is specified. The await keyword waits for this promise, so console.log prints the trimmed file content.

❓ component_behavior

intermediate

What happens if you forget to await fs.promises.readFile?

In this code snippet, what will be logged to the console?

Node.js

import { promises as fs } from 'fs';

async function readFile() {
  try {
    const data = fs.readFile('example.txt', 'utf8');
    console.log(typeof data);
  } catch (error) {
    console.log('Error:', error.message);
  }
}

readFile();

A SyntaxError due to missing await keyword

B 'string' is logged because readFile returns file content directly.

C Error: Cannot read property 'trim' of undefined

D 'object' is logged because data is a Promise object.

💡 Hint

Remember what fs.promises.readFile returns when not awaited.

Without await, fs.readFile returns a Promise object. Logging typeof data prints 'object'. The file content is not yet available.

📝 Syntax

advanced

Which option correctly reads a file using fs.promises with async/await?

Select the code snippet that correctly reads 'data.txt' and logs its content.

A

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

B

import fs from 'fs';
async function read() {
  const content = await fs.promises.readFile('data.txt');
  console.log(content.toString());
}
read();

C

import { promises as fs } from 'fs';
async function read() {
  const content = await fs.readFile('data.txt', 'utf8');
  console.log(content);
}
read();

D

import fs from 'fs/promises';
function read() {
  const content = fs.readFileSync('data.txt', 'utf8');
  console.log(content);
}
read();

💡 Hint

Check the import style and usage of async/await with fs.promises.

Option C correctly imports fs.promises as fs and uses await to read the file with encoding 'utf8'. Option C uses require which is valid in CommonJS but the prompt targets ES modules. Option C omits the 'utf8' encoding parameter, so content is a Buffer. Option C uses synchronous readFileSync inside a non-async function.

🔧 Debug

advanced

Why does this code throw an error when reading a file?

This code throws an error. What is the cause?

Node.js

import { promises as fs } from 'fs';

async function read() {
  const data = await fs.readFile('missing.txt');
  console.log(data);
}

read();

A The file 'missing.txt' does not exist, causing a rejection that is not caught.

B await cannot be used outside a try/catch block.

C Missing encoding argument causes a SyntaxError.

D fs.readFile returns undefined without encoding.

💡 Hint

What happens if you try to read a file that does not exist without error handling?

If the file does not exist, fs.promises.readFile rejects the promise. Without a try/catch, the rejection causes an unhandled promise rejection error.

🧠 Conceptual

expert

What is the advantage of using fs.promises over callback-based fs methods?

Choose the best explanation for why developers prefer fs.promises for reading files.

A fs.promises allows using async/await syntax, making asynchronous code easier to read and maintain.

B fs.promises automatically retries reading files on failure without extra code.

C fs.promises reads files synchronously, improving performance.

D fs.promises requires less memory because it streams files by default.

💡 Hint

Think about how promises improve asynchronous code structure.

fs.promises returns promises, enabling async/await syntax which simplifies asynchronous code and error handling compared to nested callbacks.