Promise catch for async errors in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch errors from the promise.

Node.js

fetchData().then(data => console.log(data)).[1](error => console.error(error));

Drag options to blanks, or click blank then click option'

Aresolve

Bthen

Cfinally

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'then' instead of 'catch' to handle errors.

Using 'finally' to catch errors (it runs regardless of success or failure).

The catch method is used to handle errors from promises.

2fill in blank

medium

Complete the code to handle errors using async/await with try/catch.

Node.js

async function load() {
  try {
    const data = await fetchData();
    console.log(data);
  } catch ([1]) {
    console.error(error);
  }
}

Drag options to blanks, or click blank then click option'

Aerror

Bdata

Cresult

Dresponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not represent an error.

Leaving the catch parameter empty.

The catch block receives the error object, commonly named error.

3fill in blank

hard

Fix the error in the promise chain to properly catch errors.

Node.js

fetchData()
  .then(data => console.log(data))
  .[1](error => console.log('Error:', error));

Drag options to blanks, or click blank then click option'

Athen

Bcatch

Cerror

Dfinally

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'then' to catch errors causes errors to be unhandled.

Using 'error' as a method name causes syntax errors.

The catch method must be used to catch errors in a promise chain.

4fill in blank

hard

Fill both blanks to handle errors and always run cleanup code.

Node.js

fetchData()
  .then(data => console.log(data))
  .[1](error => console.error(error))
  .[2](() => console.log('Cleanup done'));

Drag options to blanks, or click blank then click option'

Acatch

Bthen

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'then' instead of 'catch' for errors.

Not using 'finally' for cleanup.

Use catch to handle errors and finally to run code regardless of success or failure.

5fill in blank

hard

Fill all three blanks to create a promise chain that logs data, catches errors, and always logs completion.

Node.js

fetchData()
  .[1](data => console.log(data))
  .[2](error => console.error('Error:', error))
  .[3](() => console.log('Done'));

Drag options to blanks, or click blank then click option'

Athen

Bcatch

Cfinally

Dresolve

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of then, catch, and finally.

Using 'resolve' which is not a method on promises.

The promise chain uses then to handle success, catch for errors, and finally for code that runs always.

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