JavaScript Event Loop: An introductory manual

The event loop in JavaScript is a vital component that guarantees code execution and drives the language's asynchronous nature.

Every JavaScript developer needs to be aware of how the event loop operates. We'll go into great detail about the event loop and how it helps with asynchronous operation management in this article.

What's the purpose of an event loop?

Essentially, an event loop is a JavaScript process that manages task execution. It handles the tasks in a sequential manner after continually scanning the call stack for any outstanding issues. This prevents the main thread from being blocked and enables JavaScript to handle numerous tasks at once.

Here's a code snippet that explains the Event Loop.

1. The console.log('Start statement is executed!!!') command is executed first, which prints 'Start statement is executed!!!' to the console.
2. The setTimeout function is invoked via a callback that logs 'Statement inside the setTimeout is executed...' after 0 milliseconds. Even if the timeout is set to 0, the callback is not executed immediately.
3. The Promise.resolve().then() method generates a promise that resolves immediately and logs 'Statement inside promise is executed...' to the console.
4. Finally, the console.log('End statement is executed!!!') command is executed, which writes 'End statement is executed!!!' to the console.

How Does It Work?

1. Call Stack: When a script is executed, JavaScript begins by adding function calls to the call stack.
2. Web APIs: Asynchronous tasks, such as'setTimeout' or AJAX queries, are passed to the browser's Web APIs for execution.
3. Callback Queue: After an asynchronous task is performed, the callback is added to the callback queue.
4. Event Loop: The event loop is a continual examination of the call stack and callback queue. If the call stack is empty, it selects the first callback from the queue and adds it to the stack for execution.

Microtask Queue versus Callback Queue:

• Microtask Queue: High-priority tasks, such as promise callbacks ('then', 'catch', 'finally'), are queued and executed before the next rendering.
• Callback Queue: Regular callbacks, such as those from'setTimeout' or DOM events, are queued and performed once the microtask queue is empty.

Best practices and common pitfalls:

• Avoid Blocking the Event Loop: Long-running synchronous actions can clog the event loop, resulting in unresponsive apps. Offload large operations with asynchronous functions and Web Workers.
• Understanding Promises: A thorough understanding of promises and async/await is required for building efficient asynchronous programs. Avoid frequent errors, such as failing to manage promise rejections.

Conclusion:

The event loop is a core notion in JavaScript that allows for asynchronous programming.