How Does JavaScript Actually Make Callbacks Asynchronous?

Exploring the Mechanics of Asynchronous Callbacks

In the realm of programming, callbacks hold a crucial role in asynchronous execution. While it's widely understood that callbacks operate asynchronously, the underlying mechanism behind this behavior remains uncertain for many. This article will delve into the syntax and implementation details that enable asynchronous execution in callback-based programming.

Callback Syntax: No Implicit Indication

Contrary to popular belief, there are no specific syntactic elements that dictate whether a function will execute asynchronously or synchronously. Callbacks can function in either mode, as exemplified by the following cases:

Asynchronous:

setTimeout(function(){
    console.log("this is async");
}, 100);

Synchronous:

an_array.forEach(function(x){
    console.log("this is sync");
});

Unveiling Asynchronous Execution

JavaScript itself does not possess any inherent mechanism for asynchronous execution of functions. To achieve this behavior, one of two approaches can be taken:

1. Utilize external asynchronous functions, such as setTimeout or web workers, to handle the logic.
2. Resort to C-level implementation.

C-coded functions, like setTimeout, rely on sophisticated techniques to facilitate asynchronous execution. The primary mechanism involved is the event loop.

The Event Loop: Orchestrator of Asynchrony

At the core of every web browser lies the event loop, a remnant of asynchronous networking protocols that emerged during the early days of the internet. This mechanism allows the browser to juggle multiple I/O requests concurrently without resorting to additional threads.

The event loop relies on a pivotal system call in C known as select() (or similar variants):

select (max, readlist, writelist, errlist, timeout)

select() enables the system to monitor numerous I/O operations simultaneously. When data becomes available on any of the selected I/O channels, the function returns, signaling the browser to initiate appropriate callback execution.

The Browser's Role and Implementation

When a callback function is registered, the JavaScript interpreter stores it and concurrently invokes the select() function. Upon its return, the interpreter correlates callbacks with specific I/O channels and triggers their execution.

select() also empowers the browser to govern the invocation timing of callbacks through timeouts. By meticulously regulating the timeout argument passed to select(), callbacks can be scheduled for execution at predetermined intervals. This forms the foundation for functions like setTimeout and setInterval.

Apart from the event loop, web browsers employ web workers to facilitate asynchronous execution of JavaScript code in separate threads. However, these workers must still interconnect with the main event loop via select() to communicate back to the main thread.

Node.js and C Threads: An Alternative Perspective

Node.js, like web browsers, leverages threading for file/disk I/O operations. Upon completion of these tasks, the threads communicate with the Node.js event loop, prompting the execution of the associated callbacks.

Conclusion

The concept of asynchronous execution in callback-based programming is not directly discernible from the syntax itself. It is a result of underlying mechanisms, primarily the event loop and its interaction with C-level functions like select(). The event loop enables browsers to handle multiple I/O operations simultaneously, ensuring non-blocking execution of callbacks. Understanding these principles is crucial for grasping the intricacies of asynchronous programming and its applications across various platforms.

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