Promise in JavaScript

This content originally appeared on DEV Community and was authored by Bhuvana Sri R

Promise

A Promise in JavaScript is an object that represents the eventual completion (success) or failure of an asynchronous operation and its resulting value.

A Promise has three states:

• Pending – the operation is still going on.
• Fulfilled – the operation completed successfully (resolve).
• Rejected – the operation failed (reject).

Example :

let promise = new Promise((resolve, reject) => {
  // Do some task (like fetching data)

  let success = true; 

  if (success) {
    resolve("Task completed successfully!");
  } else {
    reject("Task failed!");
  }
});

// Using .then() and .catch()
promise
  .then(result => {
    console.log("Success:", result);
  })
  .catch(error => {
    console.log("Error:", error);
  });

Example :

function getData() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      let dataFetched = true;

      if (dataFetched) {
        resolve("Data received!");
      } else {
        reject("Failed to fetch data.");
      }
    }, 2000);
  });
}

getData()
  .then(response => console.log(response))  // Success
  .catch(error => console.log(error));      // Failure

Promise Methods :

1. – .then() → runs when promise is resolved.
2. – .catch() → runs when promise is rejected.
3. – .finally() → runs always (success or failure).
4. – Promise.all([p1, p2, …]) → waits for all promises.
5. – Promise.race([p1, p2, …]) → returns the first settled promise.

This content originally appeared on DEV Community and was authored by Bhuvana Sri R