Promise error handling in Javascript

Introduction

Promises help manage tasks that take time, like loading data. Error handling lets you catch problems if something goes wrong.

When fetching data from the internet and you want to handle failures.

When reading files or doing tasks that might fail asynchronously.

When calling APIs and you want to show a message if the call fails.

When chaining multiple tasks and need to catch errors anywhere in the chain.

Syntax

Javascript

promise.then(result => {
  // handle success
}).catch(error => {
  // handle error
});

.then() runs when the task succeeds.

.catch() runs if there is an error.

Examples

This example fetches data from the internet and logs it. If there is an error, it logs the error message.

Javascript

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.error('Error:', error));

This creates a promise that fails and catches the error to print it.

Javascript

new Promise((resolve, reject) => {
  const success = false;
  if (success) {
    resolve('Task done');
  } else {
    reject('Task failed');
  }
})
.then(message => console.log(message))
.catch(error => console.log(error));

Sample Program

This program checks if a number is greater than 10. It prints success or error messages depending on the number.

Javascript

function checkNumber(num) {
  return new Promise((resolve, reject) => {
    if (num > 10) {
      resolve('Number is greater than 10');
    } else {
      reject('Number is too small');
    }
  });
}

checkNumber(15)
  .then(message => console.log('Success:', message))
  .catch(error => console.log('Error:', error));

checkNumber(5)
  .then(message => console.log('Success:', message))
  .catch(error => console.log('Error:', error));

Important Notes

You can use .catch() at the end of a promise chain to catch any error from previous steps.

Always handle errors to avoid your program crashing silently.

Summary

Promises let you run code that takes time and handle success or failure.

Use .then() for success and .catch() for errors.

Handling errors helps keep your program working smoothly.

Practice

1. What is the main purpose of the .catch() method when working with JavaScript Promises?

easy

A. To create a new promise

B. To handle errors or rejected promises

C. To execute code after a promise resolves successfully

D. To cancel a running promise

Solution

1. Step 1: Understand the role of .catch() in Promises

   The .catch() method is designed to handle errors or rejected promises in asynchronous code.

2. Step 2: Differentiate from other methods

   .then() handles successful resolution, while .catch() specifically catches errors.

3. Final Answer:

   To handle errors or rejected promises -> Option B

4. Quick Check:

   .catch() handles errors [OK]

Hint: Remember: .catch() is for errors, .then() is for success [OK]

Common Mistakes:

• Confusing .catch() with .then()
• Thinking .catch() cancels promises
• Believing .catch() creates new promises

2. Which of the following is the correct syntax to catch errors from a promise named myPromise?

easy

A. myPromise.catch(error => console.log(error));

B. myPromise.then(error => console.log(error));

C. myPromise.error(error => console.log(error));

D. myPromise.handleError(error => console.log(error));

Solution

1. Step 1: Identify the correct method for error handling

   The correct method to catch errors from a promise is .catch().

2. Step 2: Check the syntax correctness

   The syntax myPromise.catch(error => console.log(error)); correctly attaches an error handler.

3. Final Answer:

   myPromise.catch(error => console.log(error)); -> Option A

4. Quick Check:

   Use .catch() for errors [OK]

Hint: Use .catch() after promise to handle errors [OK]

Common Mistakes:

• Using .then() to catch errors
• Using non-existent methods like .error()
• Incorrect arrow function syntax

3. What will be the output of the following code?

const p = new Promise((resolve, reject) => {
  reject('Failed');
});

p.then(result => console.log('Success:', result))
 .catch(error => console.log('Error:', error));

medium

A. Success: Failed

B. SyntaxError

C. No output

D. Error: Failed

Solution

1. Step 1: Analyze the promise rejection

   The promise immediately rejects with the message 'Failed'.

2. Step 2: Trace the .then() and .catch() handlers

   The .then() is skipped because the promise is rejected; .catch() runs and logs 'Error: Failed'.

3. Final Answer:

   Error: Failed -> Option D

4. Quick Check:

   Rejected promise triggers .catch() [OK]

Hint: Rejected promises skip .then() and go to .catch() [OK]

Common Mistakes:

• Expecting .then() to run on rejection
• Thinking no output occurs on rejection
• Confusing reject with resolve

4. Identify the error in this code snippet and choose the correct fix:

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.log('Error:', error));

// What is wrong here?

medium

A. Should use .error() instead of .catch()

B. Missing return in first .then() to pass parsed JSON

C. No error, code is correct

D. fetch() does not return a promise

Solution

1. Step 1: Review the promise chain syntax

   The code correctly chains .then() to parse JSON and then log data.

2. Step 2: Confirm error handling

   The .catch() properly handles any errors from fetch or JSON parsing.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   Proper chaining with .catch() [OK]

Hint: fetch returns promise; .then() chaining is correct [OK]

Common Mistakes:

• Thinking .then() needs explicit return (arrow with braces)
• Using .error() instead of .catch()
• Believing fetch is not promise-based

5. You want to handle errors in a promise chain but also continue processing even if an error occurs. Which pattern correctly achieves this?

fetch('https://api.example.com/data')
  .then(response => response.json())
  .catch(error => {
    console.log('Error:', error);
    return {};
  })
  .then(data => console.log('Data:', data));

hard

A. This pattern catches errors and returns a default value to continue processing

B. This pattern stops execution after an error and does not continue

C. Errors are not caught because .catch() is after .then()

D. This pattern causes an infinite loop

Solution

1. Step 1: Understand the error handling flow

   The .catch() catches any errors from the fetch or JSON parsing.

2. Step 2: Analyze the return inside .catch()

   Returning an empty object {} allows the next .then() to receive a value and continue processing.

3. Final Answer:

   This pattern catches errors and returns a default value to continue processing -> Option A

4. Quick Check:

   Return in .catch() continues chain [OK]

Hint: Return a value in .catch() to keep promise chain going [OK]

Common Mistakes:

• Assuming .catch() stops the chain without return
• Not returning a value inside .catch()
• Thinking .catch() after .then() won't catch errors