JavaScript

JavaScript - Async - Generation 2 - Promises

From
https://www.joshwcomeau.com/javascript/promises/

After problems with Callbacks were determined, a version 2 technology was released and called Promises.

Sample code

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<html>
<head>
    <meta charset="utf-8">
    <title>JavaScript - Async - Level 2 - Promises</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <meta name="description" content="JavaScript - Async - Level 2 - Promises">    
</head>
<body>
    <h1>JavaScript - Async - Level 2 - Promises</h1>
    
    <script>
        // Run JavaScript after the DOM is fully loaded
        document.addEventListener('DOMContentLoaded', function () {
            console.log('DOM fully loaded and parsed.');
            // Your script logic here
            
            console.log('Start');

            console.log('3');
            wait(1000)
            .then(() => {
                console.log('2');
                return wait(1000);
            })
            .then(() => {
                console.log('1');
                return wait(1000);
            })
            .then(() => {
                console.log('Happy New Year!!');
            });
            
            // end of event listener
        });


        function wait(duration) {
        return new Promise((resolve) => {
            setTimeout(
            () => resolve(),
            duration
            );
        });
        }
       
    </script>
</body>
</html>

Promises introduce the concept of chaining, infact the example above shows chaining multiple promises.

Promises terminology

Promises are always in one of three possible states: pending, fulfilled, and rejected.

Creating your own promise

Here is an example of creating your own promise.

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function getUser(userId) {
  return new Promise((resolve) => {
    // The asynchronous work, in this case, is
    // looking up a user from their ID
    db.get({ id: userId }, (user) => {
      // Now that we have the full user object,
      // we can pass it in here...
      resolve(user);
    });
  });
}

getUser('abc123').then((user) => {
  // ...and pluck it out here!
  console.log(user);
  // { name: 'Josh', ... }
})

the Response object.

If the code runs to completion it returns a positive object, But there could be an error.

Checkout this response object

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Response {
  ok: false,
  status: 404,
  statusText: 'Not Found',
}

error handling

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fetch('/api/get-data')
  .then((response) => {
    // ...
  })
  .catch((error) => {
    console.error(error);
  });

reject

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new Promise((resolve, reject) => {
  someAsynchronousWork((result, error) => {
    if (error) {
      reject(error);
      return;
    }
    resolve(result);
  });
});

The reject statement will mark the promise as rejected.