Promises

A Promise is an object representing the eventual result of an asynchronous operation — it may resolve (success) or reject (failure).

1. Creating a Promise

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

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

2. Consuming a Promise

promise
  .then(result => {
    console.log("Resolved:", result);
  })
  .catch(error => {
    console.log("Rejected:", error);
  });

3. Example: Simulated Async Task

function asyncTask() {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve("Task complete");
    }, 1000);
  });
}

asyncTask().then(res => console.log(res));

4. Chaining Promises

doStep1()
  .then(result1 => doStep2(result1))
  .then(result2 => doStep3(result2))
  .then(final => console.log("Final:", final))
  .catch(err => console.error(err));

5. Promise.all([])

Runs multiple promises in parallel, waits for all.

Promise.all([p1, p2, p3])
  .then(values => console.log(values))
  .catch(err => console.log(err));

6. Promise.race([])

Returns the result of the first resolved/rejected promise.

Promise.race([p1, p2])
  .then(result => console.log("First done:", result));

7. Promise States

State	Description
pending	Initial state, neither done nor failed
fulfilled	Operation completed successfully
rejected	Operation failed

8. When to Use Promises

• API calls (fetch)
• File reading
• Delayed actions
• Replacing deeply nested callbacks (callback hell)

Use async/await for cleaner syntax (covered next).