Promise catch for async errors in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the catch method do in a Promise?

The catch method handles errors or rejections that happen in the Promise chain. It lets you respond to problems without crashing your program.

beginner

How do you attach a catch to an async function call that returns a Promise?

You call the async function and then add .catch(error => { /* handle error */ }) after it to catch any errors that happen inside the async function.

beginner

Why is it important to use catch with Promises?

Without catch, errors inside Promises can go unnoticed and cause your app to behave unexpectedly or crash. catch helps keep your app stable by handling those errors.

intermediate

What happens if you forget to add catch to a Promise?

If you forget catch, unhandled Promise rejections may occur, which can cause warnings or crashes depending on your Node.js version and settings.

beginner

Show a simple example of using catch with an async function.

Example:<br><pre>async function fetchData() {
  throw new Error('Oops!');
}

fetchData()
  .catch(error =&gt; console.log('Error caught:', error.message));</pre>

What does promise.catch() do?

A Handles errors from the Promise

B Starts a new Promise

C Resolves the Promise successfully

D Cancels the Promise

catch is used to handle errors or rejections from a Promise.

Which of these is the correct way to catch errors from an async function getData()?

A getData().catch(error => console.log(error))

B catch(getData())

C getData().then(error => console.log(error))

D getData().error()

You attach .catch() directly to the Promise returned by the async function.

What happens if you don't use catch on a Promise that rejects?

A The Promise retries automatically

B The error is ignored silently

C The Promise resolves anyway

D Node.js may show an unhandled rejection warning or crash

Unhandled Promise rejections can cause warnings or crashes in Node.js.

Can catch be used to handle errors from multiple Promises chained together?

A No, each Promise needs its own catch

B Yes, one catch at the end can handle errors from the whole chain

C Only if Promises are run in parallel

D Only if Promises are synchronous

A single catch at the end of a Promise chain catches errors from any Promise in the chain.

Which Node.js version introduced warnings for unhandled Promise rejections by default?

A Node.js 6

B Node.js 8

C Node.js 10

D Node.js 12

Node.js 10 started showing warnings for unhandled Promise rejections by default.

Explain how to use catch to handle errors from an async function returning a Promise.

Think about how you would stop your app from crashing when an async call fails.

Why is it important to always add a catch when working with Promises in Node.js?

Consider what happens if errors are ignored in your code.

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