Error handling with async and await in Javascript - Time & Space Complexity

Time Complexity: Error handling with async and await

O(n)

Understanding Time Complexity

When using async and await, it's important to know how error handling affects the program's speed.

We want to see how the time to catch errors changes as the input or tasks grow.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

async function fetchData(urls) {
  const results = [];
  for (const url of urls) {
    try {
      const response = await fetch(url);
      const data = await response.json();
      results.push(data);
    } catch (error) {
      results.push(null);
    }
  }
  return results;
}

This code fetches data from multiple URLs one by one, handling errors for each fetch.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that fetches each URL sequentially.
• How many times: Once for each URL in the input array.

How Execution Grows With Input

Each URL adds one fetch and error check, so the total work grows directly with the number of URLs.

Input Size (n)	Approx. Operations
10	About 10 fetches and error checks
100	About 100 fetches and error checks
1000	About 1000 fetches and error checks

Pattern observation: The work increases evenly as the number of URLs increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to complete grows in a straight line with the number of URLs to fetch.

Common Mistake

[X] Wrong: "Using try-catch inside the loop makes the code slower exponentially."

[OK] Correct: The try-catch only adds a small fixed cost per iteration, so it grows linearly, not exponentially.

Interview Connect

Understanding how async error handling scales helps you write reliable code that performs well as tasks increase.

Self-Check

"What if we fetched all URLs in parallel using Promise.all with try-catch? How would the time complexity change?"

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