21. Creating a Promise, Chaining and Error Handling.md

Creating a Promise, Chaining and Error Handling

Creating a Promise

A Promise is created with the Promise constructor, which takes an executor function. The executor gets two functions:

• resolve(value) → marks promise as fulfilled
• reject(reason) → marks promise as rejected

Example:

const fetchData = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = true;
    if (success) {
      resolve('Data loaded successfully');
    } else {
      reject('Error loading data');
    }
  }, 1000);
});

Consuming a Promise

A promise is used with:

• .then() → when fulfilled
• .catch() → when rejected
• .finally() → runs regardless of result

fetchData
  .then((result) => console.log(result))
  .catch((error) => console.error(error))
  .finally(() => console.log('Request complete'));

Chaining Promises

Each .then() returns a new Promise, allowing you to chain async operations in sequence.

Example:

getUser(1)
  .then((user) => getPosts(user.id)) // returns promise
  .then((posts) => getComments(posts[0].id))
  .then((comments) => console.log(comments))
  .catch((err) => console.error(err));

Flow:

1. getUser resolves → passes user to next .then()
2. getPosts resolves → passes posts to next .then()
3. getComments resolves → final .then() logs data

💡 This avoids callback hell by flattening the structure.

Error Handling in Promises

Single .catch() for the whole chain

If any promise in the chain rejects, control jumps to the nearest .catch().

getUser(1)
  .then((user) => getPosts(user.id))
  .then((posts) => {
    throw new Error('Failed to load posts');
  })
  .then((posts) => getComments(posts[0].id))
  .catch((err) => console.error('Error:', err.message));

Even the thrown error inside .then() goes to .catch().

Multiple catches

You can handle errors at different points, but best practice is usually one final .catch() unless specific cases need separate handling.

Finally block

Runs regardless of success/failure — useful for cleanup.

fetchData
  .then((data) => console.log(data))
  .catch((err) => console.error(err))
  .finally(() => console.log('Finished operation'));

Interview-Ready Summary

To create a Promise, you use the new Promise constructor with an executor that calls resolve on success or reject on failure. Chaining works because each .then() returns a new Promise, passing results down the chain, which keeps async code flat and readable. Errors are handled with .catch(), which catches both rejections and exceptions thrown inside .then(). .finally() runs after completion regardless of the outcome.