promises.md

Promises

• Settled Promise: A Promise whose outcome is already determined.
• Pending Promise: A Promise whose outcome is not yet determined.
• Executor: The function passed to the new Promise constructor. It contains the instructions for the Promise.
• Promise Chaining: A technique where .then or .catch methods return a new promise, allowing these methods to be chained together.
• Unhandled Rejection: An event that is triggered when a Promise is rejected, but no error handler is attached to handle it.

Example:

let promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Done!"), 1000);
});

promise.then(
  result => console.log(result),  // "Done!"
  error => console.log(error)     // Doesn't run
);

Async/Await

• Async: A keyword that makes a function return a Promise.
• Await: A keyword that makes JavaScript wait until that Promise settles and returns its result.

Example:

async function asyncFunc() {
  let response = await fetch('https://api.github.com/users');
  let users = await response.json();
  console.log(users[0].login);
}

asyncFunc();  // Output: First username from GitHub users API

Key Differences

• Async/await syntax is cleaner and more readable compared to promise chains.
• Error handling in async/await can be done using a single try/catch block, while promises require a .catch at each level.
• Async/await makes it easier to debug with step-through debugging.
• Unlike Promises, async/await makes it easy to store the resolved value in a variable directly.