browser side event loop

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Work as a browser engineer who is expert in creating web browsers. Who is also expert in javascript.  Validate and correct below statement and also answer the question:
There are three main components on browser side javascript runtime i.e. v8, web apis, and a event loop that watches on the callback queues and executes it. 
question1 : How many type of callback queues are there in this case? What are their priority and what type of tasks they hold?
question2: If there is an express ` a = a +1` does it go to event loop or executed on v8 right away?
ChatGPT
The statement you provided is mostly accurate. However, it's important to clarify a few points.

Corrected statement: There are three main components on the browser side for JavaScript runtime: the V8 engine, Web APIs, and the event loop, which monitors the callback queues and executes the tasks.

Now let's address your questions:

Question 1: How many types of callback queues are there in this case? What are their priority, and what type of tasks do they hold?

In the context of the browser-side JavaScript runtime, there are several callback queues. The main ones are:

Task queue (also known as the "macrotask queue" or "setTimeout queue"): This queue holds tasks scheduled using setTimeout, setInterval, or similar functions. These tasks have a lower priority compared to other queues.

Microtask queue (also known as the "job queue" or "Promise queue"): This queue holds microtasks, which include promises, process.nextTick, and certain API callbacks like MutationObserver. Microtasks have a higher priority than tasks and are executed before the next rendering step.

The order of execution is as follows:

After each macrotask (task) is executed, all available microtasks are processed before the next macrotask is picked from the task queue.

Once the microtask queue is empty, the rendering process updates the UI (if necessary) before moving on to the next macrotask.

It's important to note that the event loop continuously checks both the task queue and the microtask queue, but it prioritizes the execution of microtasks over tasks.

Question 2: If there is an expression a = a + 1, does it go to the event loop or get executed in V8 right away?

The expression a = a + 1 is a simple JavaScript assignment operation and is executed in the V8 engine right away. It doesn't involve any asynchronous or non-blocking operations, so it doesn't go through the event loop or involve the callback queues.

In general, the event loop and callback queues come into play when dealing with asynchronous operations, such as AJAX requests, timers, or promises. The example you provided is a synchronous operation that executes immediately.





User
Now validate and correct below point: "So let's say there is a network call using promise. So it will handed over to web api and a callback will be register. After the data is received from network call, web api will push the callback to micro task queue of event loop?"
ChatGPT
The point you provided is partially correct. However, there are some clarifications and corrections to be made. Here's the validated and corrected version:

"When there is a network call using a promise, it will be handled by the Web API. A callback will be registered to handle the completion of the network call. Once the data is received from the network call, the callback associated with the promise will be scheduled as a microtask in the microtask queue of the event loop."

Explanation:

Network call using a promise: When a network call is made using a promise, the execution of JavaScript code is non-blocking. The promise represents the eventual completion (or failure) of the network operation.

Web API: The actual network request and its associated processing are handled by the Web API, which is provided by the browser environment. This API is responsible for handling various asynchronous operations, including network requests.

Callback registration: When the network call is initiated, a callback is registered with the Web API to handle the completion of the operation. This callback will be triggered when the network request is complete, either with the requested data or an error.

Microtask queue: Once the network request is complete and the associated callback is triggered, the callback is scheduled as a microtask in the microtask queue of the event loop.

Event loop: The event loop continuously monitors the callback queues (both task and microtask queues) and executes the queued tasks in a specific order.

Microtask execution: Microtasks have a higher priority than tasks in the event loop. Therefore, after each macrotask (task) is executed, the event loop checks if there are any microtasks in the microtask queue and executes them before moving on to the next macrotask.

By scheduling the callback associated with the network call as a microtask, it ensures that the microtask will be executed before the next macrotask, allowing for timely handling of the received data.

Overall, the network call using a promise involves the interaction between the Web API, the event loop, and the microtask queue, as described above.





User
So let's say there is two task in macro task queue and three tasks in micro task queue. After the completion of the first task from the macro task queue it will execute all the tasks in the micro task queue before picking the second task in the macro task queue?
ChatGPT
Yes, that's correct. In the event loop, after the completion of a task from the macro task queue, the event loop will process all the tasks in the micro task queue before picking the next task from the macro task queue.

The order of execution in the event loop is as follows:

The event loop picks the oldest task from the macro task queue (also known as the task queue) and executes it.

After the completion of the task, the event loop checks if there are any tasks in the micro task queue (also known as the microtask queue or job queue). If there are tasks in the micro task queue, it will execute all of them, one by one, until the micro task queue is empty.

Once the micro task queue is empty, the event loop proceeds to the next task in the macro task queue, repeating the process.

Therefore, in your scenario where there are two tasks in the macro task queue and three tasks in the micro task queue, after the completion of the first task from the macro task queue, all three tasks in the micro task queue will be executed before the event loop moves on to the second task in the macro task queue. This ensures that the micro tasks have higher priority and are processed before the next macro task is executed.





User
So when the render happen?
ChatGPT
Rendering typically occurs after the event loop has processed all the tasks and microtasks in the queue. In the browser, rendering refers to updating the user interface (UI) to reflect any changes made to the DOM (Document Object Model).

The order of operations in the event loop is as follows:

Process Tasks: The event loop picks a task from the macro task queue and executes it. These tasks can include various operations such as handling user input, network requests, or timeouts.

Process Microtasks: After a task is completed, the event loop checks if there are any microtasks in the micro task queue. If there are, it executes all the microtasks in order. Microtasks typically include promise callbacks, MutationObserver callbacks, or process.nextTick callbacks.

Render: Once the microtask queue is empty, the browser performs rendering. This step involves updating the UI to reflect any changes made to the DOM during the execution of tasks and microtasks. It includes recalculating styles, updating layout, and painting pixels on the screen.

Repeat: After rendering, the event loop goes back to step 1 and continues this cycle of processing tasks, microtasks, and rendering until there are no more tasks or microtasks in the queues.

It's important to note that rendering is not always triggered after every single task or microtask. The rendering process is typically batched to improve performance. This means that multiple changes to the DOM may be accumulated and rendered together in a single batch for efficiency purposes. The exact timing and frequency of rendering can vary based on the browser's optimization strategies and other factors.

Macro tasks (Task queue):

Network requests: Handling responses from API calls made using fetch() or XHR (XMLHttpRequest).
DOM manipulation: Performing operations like creating, updating, or removing elements from the DOM.
Timeout functions: Executing functions scheduled using setTimeout() or setInterval().
User input events: Handling events like click, keypress, or form submission.
Rendering updates: Triggering UI updates after significant changes in the application state.
IndexedDB transactions: Handling database transactions in IndexedDB.
Micro tasks (Microtask queue):

Promise callbacks: Executing callbacks associated with resolved or rejected promises.
MutationObserver callbacks: Responding to changes in the DOM structure or attributes.
process.nextTick(): Executing callbacks scheduled using Node.js's process.nextTick().
queueMicrotask(): Executing microtasks explicitly queued using the queueMicrotask() function.
Resolving async/await operations: Continuing the execution of async functions after resolving promises.