Promise

---

What is a Promise?

"Represents eventual completion of async operation"

A Promise is an object representing the eventual completion or failure of an asynchronous operation.

"A Promise is an object that represents the eventual result of an asynchronous operation, with states of pending, fulfilled, or rejected."

---

Promise States

Pending → Fulfilled or Rejected

State	Meaning
Pending	Initial state, not fulfilled or rejected
Fulfilled	Operation completed successfully
Rejected	Operation failed

"Promises have three states: pending, fulfilled (resolved), or rejected."

---

Creating Promises

new Promise(resolve, reject)

const promise = new Promise((resolve, reject) => {
  if (success) {
    resolve(value);
  } else {
    reject(error);
  }
});

"Promises are created with a constructor that takes resolve and reject callbacks."

---

Promise Methods

then · catch · finally

promise
  .then((value) => {
    // handle success
  })
  .catch((error) => {
    // handle error
  })
  .finally(() => {
    // always runs
  });

"then handles fulfillment, catch handles rejection, and finally runs regardless of outcome."

---

Promise Chaining

Return promises to chain

fetch("/api/data")
  .then((response) => response.json())
  .then((data) => processData(data))
  .then((result) => console.log(result))
  .catch((error) => console.error(error));

"Promises can be chained by returning values or new promises from then handlers."

---

Promise.all()

Wait for all, fail fast

Promise.all([promise1, promise2, promise3])
  .then((values) => {
    // all resolved
  })
  .catch((error) => {
    // any rejected
  });

• Resolves when all fulfill
• Rejects if any reject

"Promise.all waits for all promises to resolve, or rejects if any promise rejects."

---

Promise.allSettled()

Wait for all, never fails

Promise.allSettled([promise1, promise2]).then((results) => {
  // all completed (success or failure)
});

• Waits for all to complete
• Never rejects

"Promise.allSettled waits for all promises to settle, regardless of success or failure."

---

Promise.race()

First to settle wins

Promise.race([promise1, promise2]).then((value) => {
  // first to resolve/reject
});

"Promise.race resolves or rejects with the first promise to settle."

---

Async/Await

Synchronous-looking async code

async function fetchData() {
  try {
    const response = await fetch("/api/data");
    const data = await response.json();
    return data;
  } catch (error) {
    console.error(error);
  }
}

"Async/await provides a cleaner syntax for working with promises, making async code look synchronous."

---

Error Handling

try/catch with async/await

async function example() {
  try {
    const result = await promise;
  } catch (error) {
    // handle error
  }
}

Or with .catch():

promise
  .then(...)
  .catch(error => {
    // handle error
  });

---

🧠 Ultra-Short Cheat Sheet

Pending → Fulfilled/Rejected
then/catch/finally
Chaining
Promise.all (all or fail)
Promise.race (first wins)
Promise.allSettled (all complete)
async/await
Error handling