HOW event loop works in javascript

the event loop is a key mechanism in JavaScript that enables developers to write asynchronous code, handle non-blocking I/O operations, and create responsive applications without resorting to multi-threading. This is especially important in environments where JavaScript is the primary language for building user interfaces, such as web browsers.

Call Stack:

The call stack is a data structure that keeps track of the execution context of functions in the program. When you call a function, it gets pushed onto the call stack, and when the function finishes, it gets popped off the stack. This is a synchronous and single-threaded execution model.

 function first() {
    console.log("First function");
    second();
}

function second() {
    console.log("Second function");
}

first();
 ________________
|               |
|   second()    |
|   first()     |
|_______________|

2.Web APIs and Callback Queue:

When JavaScript needs to perform asynchronous tasks, it interacts with Web APIs provided by the browser or Node.js environment. Examples include the DOM API, setTimeout, fetch, and more.

When an asynchronous operation is initiated, a callback function is specified. For example, with setTimeout:

setTimeout(function() {
    console.log("Timeout callback");
}, 1000);

• setTimeout is not part of the JavaScript language; it's provided by the browser. It starts a timer, but the rest of the code keeps running.
• After approximately 1000 milliseconds, the callback function is pushed to the callback queue.

The callback queue is a data structure that holds the callback functions of completed asynchronous tasks.

. Event Loop:

The event loop continuously checks the call stack and the callback queue. If the call stack is empty, it takes the first function from the callback queue and pushes it onto the call stack.

01.Checking the Call Stack:

• The event loop checks if the call stack is empty.
• If the call stack is empty, it looks into the callback queue (task queue) .

02.Moving Callback to Call Stack:

• If there’s a callback in the queue, the event loop moves it to the call stack for execution.
• This callback represents the completion of an asynchronous task.

03.Executing Callback:

• The callback function is executed, and the call stack is modified accordingly.

04.Repeat:

• The event loop repeats this process, ensuring that asynchronous tasks are handled without blocking the main thread.

console.log("Start");

setTimeout(function () {
    console.log("Timeout callback");
}, 1000);

console.log("End");

1. “Start” is logged.
2. setTimeout is called, and the timer is started.
3. “End” is logged.
4. Approximately 1000 milliseconds later, the callback is moved to the callback queue.
5. The event loop, detecting an empty call stack, moves the callback to the call stack.
6. “Timeout callback” is logged.

!! imporatance of event loop !!

01. Single-Threaded Nature of JavaScript:

JavaScript is inherently single-threaded, meaning it has only one execution thread or call stack. This thread is responsible for executing the JavaScript code sequentially. If a particular task takes a long time to execute, it can make the entire application unresponsive, leading to a poor user experience.

2. Blocking Operations:

Certain operations, like network requests, file I/O, or heavy computations, can be time-consuming. If these operations were synchronous (blocking), they would halt the execution of the entire program until they complete. In a single-threaded environment, this would lead to a frozen or unresponsive application.

3. Asynchronous Operations and Non-Blocking I/O:

The event loop enables JavaScript to handle asynchronous operations efficiently. Asynchronous operations, such as those involving timers (setTimeout, setInterval), AJAX requests, and promises, allow code to continue executing while waiting for these operations to complete. This is achieved through callbacks and event-driven programming.

4. Web APIs and Concurrency:

When you make an asynchronous call, such as using setTimeout or fetching data from a server, JavaScript hands off the task to a Web API provided by the browser or the Node.js environment. These APIs operate in the background and don't block the main thread. Once the task is complete, a callback function is placed in the callback queue.

5. Callback Queue and Event Loop:

The event loop constantly checks the call stack and the callback queue. If the call stack is empty, it takes the first callback from the queue and pushes it onto the call stack for execution. This ensures that asynchronous tasks don’t interfere with the main execution thread.

6. Concurrency Without Multi-Threading:

Traditional multi-threading involves multiple threads running in parallel, which can lead to complex synchronization issues. JavaScript’s event loop provides a way to achieve concurrency without introducing the complexities of multi-threading. By leveraging the single-threaded nature of JavaScript and offloading time-consuming tasks to Web APIs, developers can write asynchronous code that maintains responsiveness without introducing the challenges of parallel execution.

7. Responsive User Interface:

In web development, a responsive user interface is crucial for a positive user experience. The event loop allows developers to handle tasks such as user input, animations, and network requests without freezing the UI. As a result, the application remains interactive and responsive, even when performing tasks that may take some time to complete.

Understanding the event loop is essential for writing efficient and responsive asynchronous JavaScript code, especially in environments where JavaScript is single-threaded, such as web browsers