1. Future Value Guarantees

A Promise is a built-in object representing the eventual completion or failure of an asynchronous operation.

2. Why It Matters

Promises replace nested callback spaghetti with clean, chainable asynchronous pipelines that propagate errors safely down the chain.

3. The 3 Promise States

• pending: Initial state; operation is still in progress.
• fulfilled: Operation completed successfully (resolve(value)).
• rejected: Operation failed (reject(error)).

const fetchUserData = new Promise((resolve, reject) => {
  const success = true;

  if (success) {
    resolve({ id: 101, name: "Alex" });
  } else {
    reject("Network connection failed.");
  }
});

fetchUserData
  .then((user) => {
    console.log("Loaded:", user.name);
    return user.id;
  })
  .then((id) => console.log("User ID:", id))
  .catch((err) => console.error("Error:", err));

4. Promise Combinators

• Promise.all([p1, p2]): Fails fast if ANY promise rejects. Resolves when ALL pass.
• Promise.allSettled([p1, p2]): Waits for all promises to finish regardless of success or failure.

5. Common Mistakes

• Unhandled Promise Rejections: Always attach a .catch() handler or use try...catch with async/await. Unhandled Promise rejections emit runtime warnings and can crash Node.js servers!
• Nested Promises (Pyramid of Doom): Returning a promise inside a .then() flattens the chain automatically. Do not nest .then() calls inside .then().

6. Try It Yourself

Write it yourself. The tests below run against your code.

function checkSuccess(isOnline) {
  return "";
}

7. Next Steps

Promises encapsulate async state. Next, let's make Promises look synchronous with async/await!