Tell me about promises: states, methods, async/await.

Promises in JavaScript are objects that represent the result of an asynchronous operation and can be in one of three states:

• Pending — initial state, the operation is not yet completed.
• Fulfilled — the operation completed successfully, with a result.
• Rejected — the operation failed with an error.

Main methods of promises:

• .then(onFulfilled, onRejected) — adds handlers for success and error results.
• .catch(onRejected) — adds a handler for errors (equivalent to .then(null, onRejected)).
• .finally(onFinally) — called in any case after the promise is settled.

async/await — syntactic sugar over promises, allowing writing asynchronous code in a synchronous style. A function declared with async always returns a promise. The await keyword pauses the execution of the function until the promise is settled.

Example:

function fetchData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => resolve('Data received'), 1000);
  });
}

async function getData() {
  try {
    const result = await fetchData();
    console.log(result); // 'Data received'
  } catch (error) {
    console.error('Error:', error);
  }
}

getData();

This approach simplifies reading and handling asynchronous operations.