• pending: initial state, neither fulfilled nor rejected.
• fulfilled: meaning that the operation was completed successfully.
• rejected: meaning that the operation failed.

let timeoutPromse = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Success!');
  }, 2000);
});

timeoutPromise
.then((message) => {
   alert(message);
})

timeoutPromise.then(alert);

function timeoutPromise(message, interval) {
  return new Promise((resolve, reject) => {
    if (message === '' || typeof message !== 'string') {
      reject('Message is empty or not a string!');
   } else if (interval < 0 || typeof interval !== 'number'){
      reject('Interval is negative or not a number!');
  }else{
    setTimeout(()=> {resolve(message);}, interval)
}
  })
}

timeoutPromise('Hello', 1000)
.then(message => {
  alert(message);
})
.catch(e => {
  console.log('Error: ' + e);
});

function promisifyRequest(request) {
 |     const promise = new Promise((resolve, reject) => {
 |         const unlisten = () => {
 |             request.removeEventListener('success', success);
 |             request.removeEventListener('error', error);
 |         };
 |         const success = () => {
 |             resolve(wrap(request.result));
 |             unlisten();
 |         };
 |         const error = () => {
 |             reject(request.error);
 |             unlisten();
 |         };
 |         request.addEventListener('success', success);
 |         request.addEventListener('error', error);
 |     });

 |     promise
 |         .then((value) => {
 |         // Since cursoring reuses the IDBRequest (*sigh*), we cache it for later retrieval
 |         // (see wrapFunction).
 |         if (value instanceof IDBCursor) {
 |             cursorRequestMap.set(value, request);
 |         }
 |         // Catching to avoid "Uncaught Promise exceptions"
 |     })
 |         .catch(() => { });
 |     // This mapping exists in reverseTransformCache but doesn't doesn't exist in transformCache. This
 |     // is because we create many promises from a single IDBRequest.
 |     reverseTransformCache.set(promise, request);
 |     return promise;
 | }

IndexedDB with usability.

Most modern Web APIs are promise-based, so you'll need to understand promises to get the most out of them. Among those APIs are WebRTC, Web Audio API, Media Capture and Streams, and many more. Promises will be more and more important as time goes on, so learning to use and understand them is an important step in learning modern JavaScript.

As the Promise.prototype.then() and Promise.prototype.catch() methods return promises, they can be chained.