How is asynchronous callback implemented?

Question

How do all the languages implements asynchronous callbacks?

For example in C++, one need to have a "monitor thread" to start a std::async. If it is started in main thread, it has to wait for the callback.

std::thread t{[]{std::async(callback_function).get();}}.detach();

v.s.

std::async(callback_function).get(); //Main thread will have to wait

What about asynchronous callbacks in JavaScript? In JS callbacks are massively used... How does V8 implement them? Does V8 create a lot of threads to listen on them and execute callback when it gets message? Or does it use one thread to listen on all the callbacks and keep refreshing?

For example,

setInterval(function(){},1000);
setInterval(function(){},2000);

Does V8 create 2 threads and monitor each callback state, or it has a pool thing to monitor all the callbacks?

Answer 1

V8 does not implement asynchronous functions with callbacks (including setInterval). Engine simply provides a way to execute JavaScript code.

As a V8 embedder you can create setInterval JavaScript function linked to your native C++ function that does what you want. For example, create thread or schedule some job. At this point it is your responsibility to call provided callback when it is necessary. Only one thread at a time can use V8 engine (V8 isolate instance) to execute code. This means synchronization is required if a callback needs to be called from another thread. V8 provides locking mechanism is you need this.

Another more common approach to solve this problem is to create a queue of functions for V8 to execute and use infinite queue processing loop to execute code on one thread. This is basically an event loop. This way you don't need to use execution lock, but instead use another thread to push callback function to a queue.

So it depends on a browser/Node.js/other embedder how they implement it.