CouchDB Asynchronous Programming Model (Using Callbacks and Promises)

1. Callback Functions

In JavaScript, a callback function is a traditional asynchronous programming model. It achieves this by executing callback operations within a callback function.

Benefits of Using Callback Functions

  • Can continue executing other code after an asynchronous operation completes.
  • Callback functions can be used to handle events and the results of requests.

Disadvantages of Using Callback Functions

  • May encounter callback hell (callback trap), which involves handling another callback within a callback function.
  • May increase code complexity, making it difficult to read and maintain.

2. Promise

Promise is a promise object used for handling asynchronous operations. It provides a more convenient and safer way for asynchronous programming.

Basic Concepts of Promise

  • Promise has two states: pending and resolved/faulted.
  • Promise can add a then handler function via the then() method to implement asynchronous programming.

3. Promise vs Callback Functions

| Feature | Callback Function | Promise |

| --- | --- | --- |

| Availability | Available only in modern browsers and Node.js | Available in all browsers and Node.js |

| Readability | Difficult to use, prone to callback hell | Concise and easy to use, more convenient for handling asynchronous operations |

| Error Handling | Errors need to be manually handled within the callback function | Promise can automatically push errors into the reject object |

4. Ways to Implement Promise

Promise can be implemented in the following ways:

1. Internal Implementation

  • Browser's built-in Promise object.
  • Node.js's built-in Promise object.

2. Third-Party Libraries

  • Bluebird
  • Q
  • Meteor

3. Custom Promise

  • Can create your own promise implementation.

5. Common Methods of Promise

| Method | Description |

| --- | --- |

| then() | adding a function to the promise that will be called when the promise is resolved |

| catch() | handling an exception thrown by the promise |

| finally() | running a function regardless of whether the promise was resolved or rejected |

5.1 then()

  • then(onFulfilled, onRejected): call onFulfilled with value on success and onRejected with reason on failure.

5.2 catch()

  • catch(onRejected): run onRejected if the promise is rejected.

5.3 finally()

  • finally(onFinally): run onFinally regardless of whether the promise was resolved or rejected.

5.4 all()

  • all(iterable): returns a new Promise that resolves when all in iterable have resolved.
Promise.all([p1, p2])
    .then(([result1, result2]) => {
        console.log(result1);
        console.log(result2);
    });

5.4 finaly()

  • finally(onFinally): returns a new Promise that resolves when all in iterable have resolved.
Promise.all([p1, p2])
    .then(([result1, result2]) => {
        // ... do some work ...
    })
    .finally(() => {
        console.log('All operations complete');
    });

5.4 race()

  • race(iterable): returns a new Promise that resolves as soon as one in iterable is resolved or rejected.
Promise.race([p1, p2])
    .then((result) => {
        console.log(result);
    })
    .catch((reason) => {
        console.log(reason);
    });

5.4any()

  • any(iterable): returns a new Promise that resolves as soon as one in iterable is resolved.
Promise.any([p1, p2])
    .then((result) => {
        console.log(result);
    })
    .catch((reason) => {
        console.log(reason);
    });

6. Use Cases for Promise

  • Use Promise to handle results in asynchronous operations.
  • Use Promise to handle the results of asynchronous operations in event handling.
  • In Web applications, use Promise to handle AJAX requests and other asynchronous operations.