Promises

Question 1

What is a promise?

Answer 1

It’s an object representing a value that may not be available yet, but will be at some point in the future.

It allows you to write asynchronous code in a more synchronous way.

Question 2

What are the three states of a promise?

Answer 2

• pending - start
• fulfilled - with resolved value
• rejected - with a reason of rejection

Question 3

How do you create a new promise?

Answer 3

const myPromise = new Promise(( resolve, reject ) => {});

Promise constructor and pass in a function (with 2 arg: res, rej) that defines the async operation.

Question 4

What is promise chaining? And
What are some methods for handling promises?

Answer 4

Promise chaining is a way to write multiple async operations in a sequence, like:

.then() - handle a fulfilled promise,
.catch() - handle a rejected promise,
.finally() - run code after the promise settles.

Question 5

What is async/await?

Answer 5

It’s a syntax that makes it easier to write async code using promises.

It allows you to write async code in a synchronous way, using the keywords async and await.

Question 6

What is the difference between a promise and a callback?

Answer 6

Promise - structured way to handle asynchronous operations, provide readability and organization.

Callback - function invoked after a specific event or operation completes, can result in complex and nested code.

Question 7

What is the difference between a promise and a callback?

Answer 7

Promise - structured way to handle asynchronous operations, provide readability and organization.

Callback - function invoked after a specific event or operation completes, can result in complex and nested code.

Question 8

What are the pros and cons of using Promises instead of callbacks?

Answer 8

Pros

• Avoid callback hell which can be unreadable.
• Makes it easy to write sequential asynchronous code that is readable with .then().
• Makes it easy to write parallel asynchronous code with Promise.all().
• With promises, these scenarios which are present in callbacks-only coding, will not happen:
  Call the callback too early
  Call the callback too late (or never)
  Call the callback too few or too many times
• Fail to pass along any necessary environment/parameters
• Swallow any errors/exceptions that may happen

Cons

• Slightly more complex code (debatable).
• In older browsers where ES2015 is not supported, you need to load a polyfill in order to use it.