Error handling with async and await in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using try...catch with async/await?

It helps catch errors that happen during the asynchronous operation, allowing the program to handle them gracefully instead of crashing.

beginner

How do you write an async function that handles errors?

Use async before the function, and wrap the await calls inside a try block. Use catch to handle any errors thrown.

beginner

What happens if you don't use try...catch around an await expression that rejects?

The error will be unhandled and may cause the program to crash or behave unexpectedly.

intermediate

Can you use .catch() with async/await?

Yes, you can attach .catch() to the promise returned by an async function, but using try...catch inside the async function is often clearer.

intermediate

Why is async/await preferred over .then() and .catch() for error handling?

async/await makes asynchronous code look like normal synchronous code, which is easier to read and write, especially when handling errors with try...catch.

What keyword do you use to handle errors in an async function?

A for loop

B if...else

C switch

D try...catch

try...catch blocks are used to catch errors in async functions when using await.

What happens if an awaited promise rejects and there is no try...catch?

A The program crashes or throws an unhandled rejection

B The error is ignored

C The promise resolves anyway

D The code continues silently

Without try...catch, rejected promises cause unhandled errors that can crash the program.

Which of these is a correct way to catch errors in async/await?

A await fetchData().catch(error => console.log(error))

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

C Both A and B

D Neither A nor B

Both attaching .catch() to the promise and using try...catch inside async functions are valid.

Why is try...catch preferred inside async functions over .then().catch()?

A It runs faster

B It makes code look synchronous and easier to read

C It uses less memory

D It disables errors

try...catch with async/await makes asynchronous code easier to understand by looking like normal code.

What does the 'async' keyword do?

A Marks a function to always return a promise

B Makes a function synchronous

C Prevents errors

D Runs code faster

An async function always returns a promise, allowing use of await inside it.

Explain how to handle errors when using async and await in JavaScript.

Think about how you catch errors in normal code and apply it to async code.

Describe the difference between using .catch() on a promise and using try...catch with async/await.

Consider how the code looks and where the error handling happens.

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