Promise catch for async errors in Node.js - Mini Project: Build & Apply

Handling Async Errors with Promise.catch in Node.js

📖 Scenario: You are building a simple Node.js app that fetches user data from a fake API. Sometimes the API might fail, so you need to handle errors properly.

🎯 Goal: Learn how to use Promise.catch to catch errors from asynchronous operations and handle them gracefully.

📋 What You'll Learn

Create a function that returns a Promise simulating an API call

Create a variable to hold the user ID to fetch

Call the function with the user ID and handle success and error using then and catch

Log the user data on success and log the error message on failure

💡 Why This Matters

🌍 Real World

Handling errors in asynchronous operations like API calls is essential in real-world Node.js applications to avoid crashes and provide good user experience.

💼 Career

Understanding Promise error handling is a fundamental skill for backend developers working with Node.js and asynchronous JavaScript.

1

Create a function that returns a Promise simulating an API call

Create a function called fetchUserData that takes a parameter userId and returns a new Promise. Inside the Promise, use setTimeout to simulate a delay of 100 milliseconds. If userId is 1, resolve the Promise with the object { id: 1, name: 'Alice' }. Otherwise, reject the Promise with an Error with the message 'User not found'.

Node.js

# Your code here

Hint

Use new Promise and setTimeout to simulate async behavior. Use resolve and reject inside the timeout.

2

Create a variable to hold the user ID to fetch

Create a variable called userId and set it to the number 2.

Node.js

function fetchUserData(userId) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (userId === 1) {
                resolve({ id: 1, name: 'Alice' });
            } else {
                reject(new Error('User not found'));
            }
        }, 100);
    });
}

# Your code here

Hint

Use const userId = 2; to set the user ID.

3

Call the function and handle success and error using then and catch

Call fetchUserData with the variable userId. Use then to receive the user data and log it with console.log. Use catch to catch any error and log the error message with console.log.

Node.js

function fetchUserData(userId) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (userId === 1) {
                resolve({ id: 1, name: 'Alice' });
            } else {
                reject(new Error('User not found'));
            }
        }, 100);
    });
}

const userId = 2;

# Your code here

Hint

Use fetchUserData(userId).then(...).catch(...) to handle success and errors.

4

Change the user ID to 1 and verify success handling

Change the value of the variable userId to 1 to simulate a successful API call.

Node.js

function fetchUserData(userId) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (userId === 1) {
                resolve({ id: 1, name: 'Alice' });
            } else {
                reject(new Error('User not found'));
            }
        }, 100);
    });
}

# Change userId to 1 below
const userId = 1;

fetchUserData(userId)
    .then(user => {
        console.log(user);
    })
    .catch(error => {
        console.log(error.message);
    });

Hint

Simply change const userId = 2; to const userId = 1;.

Practice

1. What is the main purpose of using catch with a Promise in Node.js?

easy

A. To log the successful result of the Promise

B. To start the asynchronous operation

C. To handle errors that occur during the asynchronous operation

D. To convert a Promise into a synchronous function

Solution

1. Step 1: Understand what catch does in Promises

   The catch method is designed to handle any errors that happen during the execution of a Promise or in the chain of Promises.

2. Step 2: Identify the role of catch in error handling

   It catches rejected Promises or exceptions thrown inside then callbacks, preventing unhandled errors.

3. Final Answer:

   To handle errors that occur during the asynchronous operation -> Option C

4. Quick Check:

   Promise catch = error handler [OK]

Hint: Remember: catch always handles errors in promises [OK]

Common Mistakes:

• Thinking catch starts the async operation
• Confusing catch with success handlers
• Believing catch makes code synchronous

2. Which of the following is the correct syntax to catch errors from a Promise named myPromise?

easy

A. myPromise.error(error => console.log(error));

B. myPromise.catch(error => console.log(error));

C. myPromise.then(error => console.log(error));

D. myPromise.finally(error => console.log(error));

Solution

1. Step 1: Recall the Promise error handling syntax

   The correct method to handle errors in a Promise is catch, which takes a callback for the error.

2. Step 2: Match the syntax with the options

   myPromise.catch(error => console.log(error)); uses catch correctly with an arrow function to log the error.

3. Final Answer:

   myPromise.catch(error => console.log(error)); -> Option B

4. Quick Check:

   Use catch for errors in Promises [OK]

Hint: Use .catch() after Promise to handle errors [OK]

Common Mistakes:

• Using then to catch errors
• Using non-existent error method
• Confusing finally with error handling

3. What will be logged to the console when running this code?

Promise.reject('Error happened')
  .catch(err => console.log('Caught:', err));

medium

A. Caught: Error happened

B. Error happened

C. Uncaught Error happened

D. Nothing is logged

Solution

1. Step 1: Understand Promise.reject and catch

   Promise.reject('Error happened') creates a rejected Promise with the error message 'Error happened'.

2. Step 2: Analyze the catch callback

   The catch method receives the error and logs it prefixed with 'Caught:'.

3. Final Answer:

   Caught: Error happened -> Option A

4. Quick Check:

   Rejected Promise caught logs error [OK]

Hint: Rejected Promise triggers catch callback [OK]

Common Mistakes:

• Expecting error to be logged without 'Caught:' prefix
• Thinking error is uncaught and crashes program
• Assuming nothing logs because of rejection

4. Identify the error in this code snippet that tries to catch a Promise rejection:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(console.error());

medium

A. The response.json() call should be inside catch

B. The then methods are missing error handlers

C. The fetch function does not return a Promise

D. The catch method is called with the result of console.error() instead of a function

Solution

1. Step 1: Check how catch is used

   The code calls console.error() immediately and passes its result (undefined) to catch, which expects a function.

2. Step 2: Understand correct catch usage

   catch should receive a function reference or arrow function, like catch(error => console.error(error)).

3. Final Answer:

   The catch method is called with the result of console.error() instead of a function -> Option D

4. Quick Check:

   Pass function to catch, not call it [OK]

Hint: Pass function to catch, don't call it immediately [OK]

Common Mistakes:

• Calling console.error() inside catch instead of passing function
• Ignoring that fetch returns a Promise
• Misplacing response.json() call

5. Given this code, what will be the output and why?

async function getData() {
  return Promise.reject('Failed to load');
}

getData()
  .then(data => console.log('Data:', data))
  .catch(err => {
    console.log('Error caught:', err);
    return 'Default data';
  })
  .then(result => console.log('Result:', result));

hard

A. Error caught: Failed to load\nResult: Default data

B. Data: Failed to load\nResult: undefined

C. Uncaught Failed to load error, program crashes

D. Only Error caught: Failed to load is logged

Solution

1. Step 1: Analyze the rejected Promise from getData

   The function returns a rejected Promise with message 'Failed to load', so the first then is skipped and control goes to catch.

2. Step 2: Understand the catch and subsequent then

   The catch logs the error and returns 'Default data', which resolves the Promise chain. The next then receives this value and logs it.

3. Final Answer:

   Error caught: Failed to load Result: Default data -> Option A

4. Quick Check:

   catch handles error and returns value for next then [OK]

Hint: Returned value from catch goes to next then [OK]

Common Mistakes:

• Thinking error stops the chain completely
• Expecting no output after catch
• Confusing rejected Promise with resolved data