Q210- What are Promises and how do they work?

A Promise in JavaScript is an object that represents the eventual completion or failure of an asynchronous operation.
It helps handle async code (like API calls, file reads, or timers) in a cleaner and more readable way than traditional callbacks.

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1️⃣ Promise States

A Promise can be in one of three states:

1. Pending → Initial state (waiting for result)
2. Fulfilled → Operation completed successfully (.then() runs)
3. Rejected → Operation failed (.catch() runs)

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2️⃣ Creating a Promise

You can create a Promise using the Promise constructor:

const myPromise = new Promise((resolve, reject) => {
  let success = true;
  
  if (success) resolve("Data fetched successfully!");
  else reject("Error fetching data!");
});

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3️⃣ Consuming a Promise

Use .then() and .catch() to handle results:

myPromise
  .then(result => console.log(result))   // Runs if resolved
  .catch(error => console.error(error)) // Runs if rejected
  .finally(() => console.log("Done!")); // Runs in both cases

Output:

Data fetched successfully!
Done!

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4️⃣ Promises with Async/Await (Cleaner Syntax)

Instead of chaining .then() and .catch(), use async/await:

async function fetchData() {
  try {
    const result = await myPromise;
    console.log(result);
  } catch (error) {
    console.error(error);
  }
}
fetchData();

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5️⃣ Promise Chaining

You can chain multiple async operations:

fetch("https://api.example.com/data")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error("Error:", err));

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💡 In Short:

A Promise is a placeholder for a value that will be available in the future —
It simplifies asynchronous code, replacing callback hell with a cleaner, structured approach.