📌 What is a Promise?

A Promise is an object in JavaScript that represents a value which may be available now, later, or never. It’s used to handle asynchronous operations.

🔁 Understanding async and await

In JavaScript, async is used to declare an asynchronous function. It always returns a Promise.

The await keyword is used inside an async function to pause execution until a Promise is resolved, making asynchronous code easier to read.

async function getData() {
  const response = await fetch("https://api.example.com/data");
  const result = await response.json();
  console.log(result);
}
getData();
    

Use Case: Ideal for handling API calls, timeouts, and non-blocking code.

🧾 Syntax of a Promise

const promise = new Promise((resolve, reject) => {
  // async task here
});
      

• resolve(): Called when the operation is successful
• reject(): Called when the operation fails

🔄 States of a Promise

• Pending: Initial state, neither fulfilled nor rejected
• Fulfilled: Operation completed successfully (calls resolve)
• Rejected: Operation failed (calls reject)

🧪 Example with Breakdown

const fetchData = () => {
  return new Promise((resolve, reject) => {
    let success = true;
    setTimeout(() => {
      if (success) {
        resolve("✅ Data fetched successfully");
      } else {
        reject("❌ Failed to fetch data");
      }
    }, 1000);
  });
};

fetchData()
  .then(response => console.log(response))
  .catch(error => console.log(error))
  .finally(() => console.log("Operation completed"));
      

🔍 Breakdown:

• fetchData() returns a Promise
• setTimeout simulates an async delay
• resolve() sends the success message
• then() handles the success output
• catch() handles the failure
• finally() always runs, success or fail

📖 Promise Methods

• then() - Handles successful resolution
• catch() - Handles rejections
• finally() - Runs after promise settles (either success or failure)
• Promise.all([]) - Waits for all promises to complete
• Promise.race([]) - Returns the result of the fastest promise
• Promise.allSettled([]) - Returns results for all (whether fulfilled or rejected)
• Promise.any([]) - Returns the first fulfilled promise (ignores rejected)

⚙️ What is async and await?

• async - Declares a function returns a Promise
• await - Waits for the result of a Promise before continuing

async function getInfo() {
  try {
    let res = await fetch("https://jsonplaceholder.typicode.com/users/1");
    let data = await res.json();
    console.log(data);
  } catch (err) {
    console.error("Error fetching:", err);
  }
}
      

Note: await can only be used inside async functions.

🧠 Summary

• Promises are better than callbacks for async tasks
• Use then/catch or async/await to handle results
• Always use catch or try/catch to prevent crashes
• finally ensures clean-up actions are performed

Promise Practice worksheet