Managing JavaScript and AJAX requests in a single thread

Many believe that Javascript is a single-threaded language, while AJAX is considered asynchronous.

Imagine this scenario;

Let's say there is a button that, when clicked, triggers an AJAX call that takes 5-6 seconds to complete. Despite this, the UI remains unblocked, allowing the user to perform other actions like clicking on another button that, in turn, starts executing some code. In such a situation, when does the AJAX callback function get executed? Does it have to wait for the other code to finish, or can it run in a parallel thread?

Answer №1

Events are lined up in a queue, meaning that the handler for an Ajax call will only run on the event loop after the call completes. The single thread will move on to the next event in the queue once it finishes processing your button handler. This implies that you may need to wait for the code triggered by the button click to complete, unless, of course, the Ajax request finishes prior to the button click, in which case the button click handler would need to wait. The optimal approach is to break down your algorithm into smaller sections that can be added to the queue using setTimeout, although this can be quite challenging.

Answer №2

After conducting some research on the topic, I've come to realize that javascript operates more like a queue system rather than being multi-threaded as I had previously thought.

To put it simply, the AJAX callback may need to wait for the completion of a `click` event depending on precise timing. It could also be required to wait for any other code that was executed at the same time.

This is why functions like `while(true)` or `alert()` can halt all scripts running on a website simultaneously.

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