Using promises

The traditional way to handle an asynchronous operation is to use callback functions for the success or failure of the operation. One of the callback functions is called, depending on the result of the call. The following example shows the idea of using the callback function:

function doAsyncCall(success, failure) {
    // Do some api call
    if (SUCCEED)
        success(resp);
    else
        failure(err);
}

success(response) {
    // Do something with response
}

failure(error) {
    // Handle error
}

doAsyncCall(success, failure);

A promise is an object that represents the result of an asynchronous operation. The use of promises simplifies the code when executing asynchronous calls. Promises are non-blocking.

A promise can be in one of three states:

• Pending: Initial state
• Fulfilled: Successful operation
• Rejected: Failed operation

With promises, we can execute asynchronous...