Error handling with async and await in Javascript - Mini Project: Build & Apply

Error handling with async and await

📖 Scenario: You are building a simple web app that fetches user data from a server. Sometimes the server might fail or the data might not be available. You want to handle these errors gracefully so the app does not crash and shows a friendly message.

🎯 Goal: Learn how to use async and await to fetch data and handle errors using try and catch blocks in JavaScript.

📋 What You'll Learn

Create an async function to simulate fetching user data

Add a variable to simulate success or failure

Use try and catch inside the async function to handle errors

Print the result or error message

💡 Why This Matters

🌍 Real World

Handling errors when fetching data from servers is common in web apps to avoid crashes and show friendly messages.

💼 Career

Knowing async error handling is essential for frontend and backend developers working with APIs and asynchronous code.

1

Create an async function to fetch user data

Write an async function called fetchUserData that returns a resolved promise with the string 'User data loaded'.

Javascript

# Your code here
// Write an async function called fetchUserData that returns 'User data loaded'

Hint

Use the async keyword before the function name and return a string directly.

2

Add a variable to simulate success or failure

Inside the fetchUserData function, create a variable called success and set it to false to simulate a failure scenario.

Javascript

async function fetchUserData() {
    // Add a variable called success and set it to false
    // Your code here
    return 'User data loaded';
}

Hint

Use const success = false; inside the function.

3

Use try and catch to handle errors

Modify the fetchUserData function to use a try block. Inside try, if success is false, throw a new Error with the message 'Failed to load user data'. Otherwise, return 'User data loaded'. Add a catch block that returns the error message.

Javascript

async function fetchUserData() {
    const success = false;
    // Use try and catch to handle errors
    // Your code here
}

Hint

Use try { if (!success) throw new Error(...); return ...; } catch (error) { return error.message; }

4

Call the async function and print the result

Write code to call fetchUserData() using await inside an async function called main. Then print the result using console.log. Finally, call the main function.

Javascript

async function fetchUserData() {
    const success = false;
    try {
        if (!success) {
            throw new Error('Failed to load user data');
        }
        return 'User data loaded';
    } catch (error) {
        return error.message;
    }
}

// Your code here
// Create async function main, call fetchUserData with await, print result, then call main

Hint

Wrap the call in an async main function, use await, then print the result.

Practice

1. What is the main purpose of using try...catch with async and await in JavaScript?

easy

A. To handle errors that occur during asynchronous operations

B. To make the code run faster

C. To convert asynchronous code into synchronous code

D. To declare variables inside async functions

Solution

1. Step 1: Understand async/await behavior

   Async functions return promises, and await pauses execution until the promise settles.

2. Step 2: Role of try...catch with async/await

   Errors in awaited promises can be caught using try...catch to prevent program crashes.

3. Final Answer:

   To handle errors that occur during asynchronous operations -> Option A

4. Quick Check:

   Error handling = To handle errors that occur during asynchronous operations [OK]

Hint: Use try...catch around await to catch async errors [OK]

Common Mistakes:

• Thinking async/await makes code synchronous
• Ignoring errors from awaited promises
• Using try...catch without async functions

2. Which of the following is the correct syntax to catch errors from an async function using await?

easy

A. try await asyncFunc() catch error { console.log(error); }

B. await try { asyncFunc(); } catch (error) { console.log(error); }

C. asyncFunc().catch(error => console.log(error));

D. try { await asyncFunc(); } catch (error) { console.log(error); }

Solution

1. Step 1: Identify correct try...catch syntax

   try...catch blocks wrap code that might throw errors; await must be inside try.

2. Step 2: Check each option's syntax

   try { await asyncFunc(); } catch (error) { console.log(error); } correctly wraps await in try and catches errors; others have syntax errors or misuse await.

3. Final Answer:

   try { await asyncFunc(); } catch (error) { console.log(error); } -> Option D

4. Quick Check:

   Correct try...catch with await = try { await asyncFunc(); } catch (error) { console.log(error); } [OK]

Hint: Put await inside try block to catch errors [OK]

Common Mistakes:

• Placing await outside try block
• Using incorrect try...catch syntax
• Confusing promise.catch with try...catch

3. What will be the output of the following code?

async function fetchData() {
  try {
    let result = await Promise.reject('Failed');
    console.log('Success:', result);
  } catch (error) {
    console.log('Error:', error);
  }
}
fetchData();

medium

A. Success: Failed

B. Error: Failed

C. Uncaught (in promise) Failed

D. No output

Solution

1. Step 1: Analyze Promise.reject inside try block

   Promise.reject('Failed') creates a rejected promise; await pauses until rejection.

2. Step 2: Catch block handles the rejection error

   The rejection triggers catch block, printing 'Error: Failed'.

3. Final Answer:

   Error: Failed -> Option B

4. Quick Check:

   Rejected promise caught = Error: Failed [OK]

Hint: Rejected promises inside try trigger catch block [OK]

Common Mistakes:

• Expecting success message on rejection
• Missing catch block causes unhandled rejection
• Confusing Promise.reject with Promise.resolve

4. Identify the error in this code snippet:

async function load() {
  try {
    const data = await fetch('https://api.example.com/data');
  } catch {
    console.log('Fetch failed');
  }
}
load();

medium

A. Missing error parameter in catch block

B. fetch cannot be awaited

C. async keyword is missing

D. try block cannot contain await

Solution

1. Step 1: Check catch block syntax

   Catch block should have an error parameter to receive the error object.

2. Step 2: Confirm other parts are correct

   fetch returns a promise and can be awaited; async keyword is present; try can contain await.

3. Final Answer:

   Missing error parameter in catch block -> Option A

4. Quick Check:

   Catch needs error parameter = Missing error parameter in catch block [OK]

Hint: Always include error parameter in catch(e) [OK]

Common Mistakes:

• Omitting error parameter in catch
• Thinking fetch is synchronous
• Removing async keyword by mistake

5. You want to fetch user data and then fetch posts for that user. If either fetch fails, you want to log the error and continue without stopping the program. Which code correctly handles errors using async/await?

hard

A. async function getUserAndPosts() {
try {
const user = await fetch('/user');
const posts = await fetch(`/posts?userId=${user.id}`);
} catch (e) {
console.log('Error:', e);
}
}

B. async function getUserAndPosts() {
const user = await fetch('/user').catch(e => console.log('User fetch error:', e));
const posts = await fetch(`/posts?userId=${user.id}`).catch(e => console.log('Posts fetch error:', e));
}

C. async function getUserAndPosts() {
try {
const user = await fetch('/user');
} catch (e) {
console.log('User fetch error:', e);
}
try {
const posts = await fetch(`/posts?userId=${user.id}`);
} catch (e) {
console.log('Posts fetch error:', e);
}
}

D. async function getUserAndPosts() {
const user = fetch('/user');
const posts = fetch(`/posts?userId=${user.id}`);
console.log('Data fetched');
}

Solution

1. Step 1: Understand error handling for multiple awaits

   Separate try...catch blocks allow handling errors individually without stopping the whole function.

2. Step 2: Analyze each option

   async function getUserAndPosts() { try { const user = await fetch('/user'); } catch (e) { console.log('User fetch error:', e); } try { const posts = await fetch(`/posts?userId=${user.id}`); } catch (e) { console.log('Posts fetch error:', e); } } uses separate try...catch for user and posts fetch, logging errors and continuing execution. async function getUserAndPosts() { try { const user = await fetch('/user'); const posts = await fetch(`/posts?userId=${user.id}`); } catch (e) { console.log('Error:', e); } } stops on first error. async function getUserAndPosts() { const user = await fetch('/user').catch(e => console.log('User fetch error:', e)); const posts = await fetch(`/posts?userId=${user.id}`).catch(e => console.log('Posts fetch error:', e)); } mixes await with .catch but can cause undefined user. async function getUserAndPosts() { const user = fetch('/user'); const posts = fetch(`/posts?userId=${user.id}`); console.log('Data fetched'); } lacks await and error handling.

3. Final Answer:

   Option C correctly handles errors separately and continues execution -> Option C

4. Quick Check:

   Separate try...catch for each await = async function getUserAndPosts() { try { const user = await fetch('/user'); } catch (e) { console.log('User fetch error:', e); } try { const posts = await fetch(`/posts?userId=${user.id}`); } catch (e) { console.log('Posts fetch error:', e); } } [OK]

Hint: Use separate try...catch blocks to handle multiple awaits safely [OK]

Common Mistakes:

• Using one try...catch for multiple awaits stopping on first error
• Mixing await with .catch causing undefined variables
• Not awaiting fetch calls