Promise catch for async errors in Node.js - Performance & Optimization

Performance: Promise catch for async errors

MEDIUM IMPACT

This concept affects how asynchronous errors are handled without blocking the event loop or causing unhandled promise rejections.

Handling errors in asynchronous code

Node.js

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    return data;
  } catch (error) {
    console.error('Fetch failed:', error);
    return null;
  }
}

fetchData();

Catches errors properly, preventing unhandled rejections and keeping the event loop stable.

📈 Performance GainPrevents process crashes and memory leaks, improves responsiveness

Handling errors in asynchronous code

Node.js

async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return data;
}

fetchData(); // No catch or error handling

No error handling causes unhandled promise rejections, which can crash the Node.js process or cause unpredictable behavior.

📉 Performance CostBlocks error propagation, may cause process crash or memory leaks

Performance Comparison

Pattern	Error Handling	Event Loop Impact	Process Stability	Verdict
No .catch() on Promise	None	Unhandled rejection may crash process	May crash process	[X] Bad
Using .catch() on Promise	Proper	Keeps event loop free	Stable process	[OK] Good
Async/await without try/catch	None	Unhandled rejections possible	Unstable	[X] Bad
Async/await with try/catch	Proper	Stable event loop	Stable process	[OK] Good

Rendering Pipeline

In Node.js, promise error handling affects the event loop and error propagation rather than browser rendering. Proper .catch() usage prevents unhandled rejections that can crash the process.

→Event Loop

→Error Propagation

⚠️ BottleneckUnhandled promise rejections can cause process crashes.

Core Web Vital Affected

INP

This concept affects how asynchronous errors are handled without blocking the event loop or causing unhandled promise rejections.

Optimization Tips

1Always attach .catch() to every Promise chain to handle errors.

2Use try/catch blocks with async/await to catch asynchronous errors.

3Unhandled promise rejections can crash your Node.js process.

Performance Quiz - 3 Questions

What happens if you forget to add .catch() to a Promise chain in Node.js?

A Unhandled promise rejections may crash the process or cause warnings.

B The Promise will automatically retry the operation.

C The event loop will speed up.

D The Promise will silently succeed without errors.

DevTools: Node.js --inspect with Chrome DevTools or built-in inspector

How to check: Run Node.js with --inspect flag, open DevTools, go to Console panel, and watch for unhandled promise rejection warnings or errors.

What to look for: Look for 'UnhandledPromiseRejectionWarning' or error stack traces indicating missing .catch() handlers.

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