Promises

What is a Promise?

A Promise is an object representing an asynchronous operation (that takes time) and its future result.
It can be in three states:

• pending → waiting (the promise has not settled yet)
• fulfilled → successfully resolved (resolve)
• rejected → failed (reject)

➡ Instead of blocking the code, a Promise allows you to wait for the result later.

---

Creating a Promise

let promise = new Promise((resolve, reject) => {
  let success = true;

  if (success) {
    resolve("Success");
  } else {
    reject("Error");
  }
});

promise
  .then(result => console.log(result))   // on success
  .catch(error => console.error(error))  // on error
  .finally(() => console.log("Promise settled")); // always executed

---

Example with setTimeout

function wait(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

wait(1000).then(() => console.log("1 second passed"));

---

Concrete example: simulated request

function getUser() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      let success = true;
      if (success) {
        resolve({ name: "Alice", age: 25 });
      } else {
        reject("Unable to fetch user");
      }
    }, 2000);
  });
}

getUser()
  .then(user => console.log("User:", user))
  .catch(err => console.error("Error:", err));

---

Best practices

• Always handle errors with .catch() (or with try/catch if using async/await).
• Use .finally() to run code regardless of outcome (success or error).
• Prefer async/await for more readable and maintainable code when chaining promises.