Explain callback queue (Task Queue) and Microtask queue (Job Queue)

Callback Queue (Task Queue):

• The callback queue, also known as the task queue, is where asynchronous operations such as event callbacks, setTimeout, setInterval, and I/O operations are placed after they are executed.
• Callbacks in the callback queue are processed in the order in which they were added, following the event loop's FIFO (First-In-First-Out) principle.
• The event loop continuously checks the callback queue for tasks to execute. When the call stack is empty, the event loop takes tasks from the callback queue and pushes them onto the call stack for execution.

Microtask Queue (Job Queue):

• The microtask queue, also known as the job queue, is used for executing microtasks, which are typically short-lived tasks that need to be executed before rendering or other long-running tasks.
• Microtasks include promises (resolved or rejected), mutation observer callbacks, and queueMicrotask API.
• Unlike the callback queue, microtasks have higher priority and are executed before any callback from the callback queue.
• Microtasks are executed after each task (macrotask) completes but before the next task is picked from the callback queue.

In summary, while both callback queue and microtask queue manage asynchronous operations, they have different priorities and execution orders. Callback queue handles regular asynchronous tasks, while the microtask queue handles microtasks with higher priority, ensuring faster execution for critical operations like promise resolution and DOM updates.

Example of Callback Queue (Task Queue):

// Example of callback queue (task queue)

console.log("Start");

// Asynchronous operation using setTimeout
setTimeout(() => {
  console.log("Inside setTimeout callback");
}, 0);

console.log("End");


//Output:
Start
End
Inside setTimeout callback

Explanation:

• Initially, "Start" is logged to the console.
• Then, the setTimeout function is called with a callback function. The setTimeout callback is added to the callback queue.
• "End" is logged to the console.
• Once the call stack is empty, the event loop picks up the setTimeout callback from the callback queue and executes it, logging "Inside setTimeout callback" to the console.

Example of Microtask Queue (Job Queue):

// Example of microtask queue (job queue)

console.log("Start");

// Asynchronous operation using Promise
Promise.resolve().then(() => {
  console.log("Inside Promise microtask");
});

console.log("End");


//Output:
Start
End
Inside Promise microtask

Explanation:

• "Start" is logged to the console.
• A Promise is created using Promise.resolve() with a .then() method, which adds a microtask to the microtask queue.
• "End" is logged to the console.
• Once the call stack is empty, the event loop picks up the microtask from the microtask queue and executes it, logging "Inside Promise microtask" to the console.

When both setTimeout and Promises are used in the same code, it's essential to understand how they interact with each other in terms of execution order and priority. In general, Promises scheduled using then() will be executed before the callback functions scheduled with setTimeout, regardless of the timeout duration. This is because Promises are processed in the microtask queue, which has a higher priority than the callback queue (task queue) where setTimeout callbacks reside.

Here's an example illustrating this behavior:

console.log("Start");

// Asynchronous operation using Promise
Promise.resolve().then(() => {
  console.log("Inside Promise microtask");
});

// Asynchronous operation using setTimeout
setTimeout(() => {
  console.log("Inside setTimeout callback");
}, 0);

console.log("End");


//Output:

Start
End
Inside Promise microtask
Inside setTimeout callback

Explanation:

• "Start" is logged to the console.
• A Promise is created using Promise.resolve() with a .then() method, which adds a microtask to the microtask queue.
• "End" is logged to the console.
• Once the call stack is empty, the event loop picks up the microtask from the microtask queue and executes it, logging "Inside Promise microtask" to the console.
• After that, the setTimeout callback is executed, logging "Inside setTimeout callback" to the console.

In summary, when using both setTimeout and Promises in the same code, Promises will be executed first due to their higher priority in the microtask queue.

In JavaScript, starvation in the context of callback queue and microtask queue typically refers to situations where one type of task (callbacks or microtasks) monopolizes the event loop, preventing the execution of tasks from the other queue. This can lead to delays or indefinite blocking of tasks, impacting the responsiveness and performance of the application.

Callback Queue Starvation:

• If the callback queue (also known as the task queue) is constantly filled with tasks, such as asynchronous I/O operations or timer callbacks, it may prevent microtasks from being executed promptly.
• As a result, microtasks, such as promises and mutation observer callbacks, may be delayed or blocked from executing until the callback queue is emptied.
• This can lead to delayed processing of important microtasks, impacting the application's responsiveness and user experience.

Microtask Queue Starvation:

• Conversely, if the microtask queue is constantly filled with microtasks, such as promise callbacks or mutation observer callbacks, it may prevent tasks from the callback queue from being executed promptly.
• As a result, tasks queued in the callback queue, such as timer callbacks or I/O operations, may be delayed or blocked from executing until the microtask queue is emptied.
• This can lead to delayed processing of important tasks, impacting the application's responsiveness and performance.

To mitigate starvation in JavaScript:

• Balanced Task Execution: Ensure a balanced execution of both tasks and microtasks to prevent one queue from monopolizing the event loop.
• Optimize Task Queuing: Prioritize critical tasks and microtasks to ensure timely execution of high-priority operations.
• Minimize Long-Running Tasks: Avoid synchronous operations or long-running tasks that may block the event loop and cause starvation.
• Use Microtasks Wisely: Be mindful of the performance implications of microtasks and avoid excessive microtask queuing that may starve the callback queue.

By understanding the behavior of the event loop and carefully managing the execution of tasks and microtasks, developers can mitigate the risk of starvation and ensure smooth and responsive JavaScript applications.

let's consider an example where starvation occurs due to excessive microtask queuing, preventing tasks in the callback queue from executing promptly.

// Function to create a microtask
function createMicrotask(index) {
    return () => {
        console.log(`Microtask ${index} executed`);
    };
}

// Create an array of microtasks
const microtasks = [];
for (let i = 1; i <= 10; i++) {
    microtasks.push(createMicrotask(i));
}

// Queue the microtasks
for (const task of microtasks) {
    Promise.resolve().then(task);
}

// Queue a callback task
setTimeout(() => {
    console.log('Callback task executed');
}, 0);

In this example:

1. We create an array of microtasks using a loop, each containing a logging function.
2. We queue each microtask using Promise.resolve().then(task).
3. Additionally, we queue a callback task using setTimeout with a timeout of 0 milliseconds.

Now, since microtasks have higher priority than tasks in the callback queue, the microtasks will be executed before the callback task. However, if there are too many microtasks queued up, they may monopolize the event loop, causing the callback task to be delayed or starved.

If there are a large number of microtasks (e.g., thousands or more), they may take a significant amount of time to execute, causing the callback task to be delayed indefinitely. This scenario demonstrates starvation in JavaScript, where excessive queuing of microtasks prevents the timely execution of tasks from the callback queue.