Promises

Promise States

• Pending
• Resolved
• Rejected

What Is a Promise?

• A promise is a placeholder that gets replaced by a future value.
• It is a container for a future value.
• It is an object representing the eventual completion or failure of an asynchronous operation.
• Promises solve inversion of control: instead of passing a callback to someone else, you get an object back that you control.

During promise chaining, we need to return data because we pipe the data to the next callback.

function creatOrder(cart) {
  const pr = new Promise(function (resolve, reject)) {
  if (!validateCart(cart))
  const err = new Error("Cart is not valid")
    reject(err)
  } else {
    const orderId = dbCall.getOrderId()
    if (orderId) {
      resolve(orderId)
    }
  }

  return pr;
}

• If there is a .then() method after the .catch(), it will definitely be called.
• resolve and reject can only effectively be called once.
• With a promise, we attach a callback; we do not call it.

Promise APIs

Promise.all()

• Takes an iterable, such as an array.
• Returns an array output.
• If any promise fails, it throws an error (returns the first rejection reason) as soon as possible. It does not wait for other promises to complete.

const p1 = new Promise(() => {});
const p2 = new Promise(() => {});
const p3 = new Promise(() => {});
Promise.all([p1, p2, p3]).then((res) => {
  console.log(res);
});

Promise.allSettled()

• Returns even if any promise gets rejected.

const p1 = new Promise(() => {});
const p2 = new Promise(() => {});
const p3 = new Promise(() => {});
Promise.allSettled([p1, p2, p3]).then((res) => {
  console.log(res);
});

Promise.race()

• Returns the value of the first settled promise.
• If the first promise fails, an error is returned.

const p1 = new Promise(() => {});
const p2 = new Promise(() => {});
const p3 = new Promise(() => {});
Promise.race([p1, p2, p3]).then((res) => {
  console.log(res);
});

Promise.any()

• Same as Promise.race(), but returns the result of the first resolved promise.
• Rejects only if all reject, with an AggregateError containing multiple errors inside one error object.

const p1 = new Promise(() => {});
const p2 = new Promise(() => {});
const p3 = new Promise(() => {});
Promise.any([p1, p2, p3]).then((res) => {
  console.log(res);
});