Promises in JavaScript are a way to handle asynchronous operations. They represent the eventual completion or failure of an asynchronous task and allow us to write cleaner and more readable code.
A promise is an object that may produce a single value in the future, either a resolved value or a reason for rejection. It has three states: pending, fulfilled, or rejected. When a promise is pending, it means the asynchronous operation is still in progress. Once the operation is completed successfully, the promise is fulfilled with a value. If the operation fails, the promise is rejected with a reason for the failure.
Promises provide a set of methods to handle the fulfillment or rejection of the asynchronous task. These methods include then(), catch(), and finally(). The then() method is used to handle the fulfillment of the promise, the catch() method is used to handle the rejection, and the finally() method is used to execute code regardless of the promise’s outcome.
Here’s an example of using promises in JavaScript:
// Asynchronous function that returns a promise function fetchData() { return new Promise((resolve, reject) => { setTimeout(() => { const data = 'Some data'; // Simulating a successful operation resolve(data); // Simulating a failed operation // reject('Error occurred'); }, 2000); }); } // Using the promise fetchData() .then((data) => { console.log('Data:', data); }) .catch((error) => { console.error('Error:', error); }) .finally(() => { console.log('Promise completed'); });
In the above example, the fetchData() function returns a promise that resolves with the data after a delay of 2 seconds. We can use the then() method to handle the successful fulfillment of the promise and the catch() method to handle any errors that occur. The finally() method is used to execute code that should run regardless of the promise’s outcome.
This is a concise and structured explanation of promises in JavaScript.