How can I execute three asynchronous HTTP calls sequentially using jQuery Promises?

Chaining Asynchronous Calls with jQuery Promises

Asynchronous programming in JavaScript allows code to execute without blocking the user's interface. To achieve this, promises are often used to manage the flow of data and ensure that tasks are executed in a specific order.

In the given scenario, you aim to perform three asynchronous HTTP calls in sequence. You've provided a partial solution involving two functions with deferred promises. To extend it to three functions, let's delve into the core concept.

In each function that initiates an asynchronous request, return the jqXHR object returned by $.ajax(). These jqXHR objects are Promise-compatible, which means they can be easily chained using .then(), .done(), .fail(), or .always().

In this case, .then() is the most suitable as it allows you to connect the three calls and pass data from one to the other. The following code sample illustrates the complete solution:

<code class="javascript">function first() {
  return $.ajax(...);
}

function second(data, textStatus, jqXHR) {
  return $.ajax(...);
}

function third(data, textStatus, jqXHR) {
  return $.ajax(...);
}

function main() {
  first().then(second).then(third);
}</code>

The arguments data, textStatus, and jqXHR are passed to each subsequent function from the previous call's $.ajax() call. This enables you to pass data from first() to second(), and from second() to third(), ensuring that the calls are executed synchronously and the data flows seamlessly between them.

You can also use a helper function like $.when('foo') to deliver a fulfilled promise in place of $.ajax(...) during testing or while developing your solution.

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