Question 22

Question

What is the difference between .then() and .catch() in promise chains?

Answer

.then() and .catch() are methods used in promise chains to handle different outcomes of a Promise. They let you define what happens when a Promise successfully resolves or rejects:

1. .then(onFulfilled):

• Purpose: Executed when the Promise is fulfilled (resolves successfully).

• Parameter onFulfilled: A function that receives the result of the fulfilled Promise as its argument. This function will be called with the resolved value if everything goes as expected.

• Example:

  const promise = new Promise((resolve, reject) => {
      setTimeout(() => resolve("Success!"), 1000); // Simulate an asynchronous operation
  });
  
  promise
      .then((result) => {
          console.log(result); // Output: "Success!" after 1 second
      });

2. .catch(onRejected):

• Purpose: Executed when the Promise is rejected (an error occurs).

• Parameter onRejected: A function that receives the reason for the rejection as its argument. This function will be called if something goes wrong during the asynchronous operation.

• Example:

  const promise = new Promise((resolve, reject) => {
      setTimeout(() => reject(new Error("Something went wrong!")), 1000);
  });
  
  promise
      .catch((error) => {
          console.error(error); // Output: Error message after 1 second
      });

Key Points:

• You can chain multiple .then() calls to perform sequential actions after a successful resolution.

• The catch block is crucial for graceful error handling in asynchronous code.