How JavaScript Prioritizes Microtasks Over Tasks in the Event Loop

In the context of JavaScript's event loop, understanding how tasks and microtasks are prioritized is key to knowing the order of execution. Here’s a detailed explanation of how the system distinguishes and prioritizes these tasks:

Tasks and Microtasks

1. Tasks: These are scheduled by events like setTimeout, setInterval, or I/O operations (e.g., the completion of a fetch request).
2. Microtasks: These include promises (resolved/rejected), MutationObserver callbacks, and queue microtasks using queueMicrotask.

Event Loop Execution Order

The JavaScript event loop handles tasks and microtasks as follows:

1. Execute any tasks from the task queue (e.g., a fetch request completion).
2. After executing a task, the event loop processes all microtasks in the microtask queue before moving on to the next task.

How the System Prioritizes Microtasks Over Tasks

When the event loop runs, it follows a specific sequence:

1. Task Execution: It takes the first task from the task queue and executes it.
2. Microtask Queue Check: After executing a task, the event loop checks the microtask queue. If there are any microtasks, it executes all of them before proceeding to the next task.

Example

To illustrate, consider a scenario where a fetch operation completes (a task), and a promise resolution (a microtask) is queued:

fetch('https://example.com/data')
  .then(response => response.json())
  .then(data => console.log('Fetch complete:', data)); // Microtask

setTimeout(() => console.log('setTimeout callback'), 0); // Task

Promise.resolve().then(() => console.log('Promise resolved')); // Microtask

console.log('Script end'); // Synchronous        

Execution Steps

1. Synchronous Code: Runs first.
2. Tasks and Microtasks Queued:
3. Task Execution:
4. Microtask Queue:

Output Sequence

Given the priority rules:

1. Script end (synchronous)
2. Promise resolved (microtask)
3. Fetch complete: <data> (microtask from fetch)
4. setTimeout callback (task)

How the System Distinguishes Them

1. Microtasks are enqueued during the execution of the current task or microtask. They are placed in the microtask queue.
2. Tasks are enqueued as a result of external events like timers or I/O operations and are placed in the task queue.

The system doesn't need explicit tagging of tasks and microtasks; it implicitly knows the origin:

• Microtasks come from promise resolutions or queueMicrotask.
• Tasks come from events like setTimeout, setInterval, or completion of asynchronous operations like fetch.

Conclusion

The event loop ensures that microtasks are always executed before the next task in the task queue. By maintaining separate queues and processing microtasks immediately after the current task, the system prioritizes microtasks without needing to explicitly identify them beyond their source of scheduling.