Microtasks

JavaScript Essentials: Asynchronous JS - Microtasks

This document dives into the crucial concept of Microtasks in JavaScript's asynchronous execution model. Understanding microtasks is key to predicting the order in which asynchronous operations complete and how your code behaves.

What are Microtasks?

Microtasks are tasks that are queued to be executed after the current JavaScript execution context (call stack) is empty, but before the browser re-renders or handles other events. They are a core part of the Event Loop.

Key Characteristics:

• Higher Priority than Macrotasks: Microtasks are processed before macrotasks (like setTimeout, I/O events, etc.).
• Queued: Microtasks are placed in a Microtask Queue.
• Non-Blocking: They allow the main thread to remain responsive while asynchronous operations are handled.
• Examples:
  • Promises (.then(), .catch(), .finally())
  • queueMicrotask() (less common, but explicitly adds a task to the microtask queue)
  • MutationObserver (used for observing changes in the DOM)

How Microtasks Work with the Event Loop

The Event Loop is the engine that drives asynchronous JavaScript. Here's a simplified breakdown of how microtasks fit in:

1. Call Stack: JavaScript executes code in a Call Stack (LIFO - Last In, First Out).
2. Microtask Queue: Asynchronous operations (like resolving a Promise) place callbacks into the Microtask Queue.
3. Event Loop Iteration:
   • After the Call Stack is empty, the Event Loop checks the Microtask Queue.
   • It takes the first task from the Microtask Queue and pushes it onto the Call Stack for execution.
   • This process repeats until the Microtask Queue is empty.
4. Macrotask Queue: Once the Microtask Queue is empty, the Event Loop checks the Macrotask Queue (e.g., setTimeout, I/O).
5. Rendering: The browser has a chance to re-render the page after each macrotask.

Visual Representation:

+-----------------+     +-----------------+     +-----------------+
|   Call Stack    | --> | Microtask Queue | --> |  Macrotask Queue |
+-----------------+     +-----------------+     +-----------------+
                                ^
                                |
                                Event Loop

Example: Promises and Microtasks

console.log('Start');

Promise.resolve().then(() => {
  console.log('Promise resolved');
});

console.log('End');

Output:

Start
End
Promise resolved

Explanation:

1. console.log('Start') is executed and added to the Call Stack, then removed.
2. Promise.resolve() creates a resolved Promise. The .then() callback is added to the Microtask Queue.
3. console.log('End') is executed and added to the Call Stack, then removed.
4. The Call Stack is now empty. The Event Loop checks the Microtask Queue.
5. The .then() callback (console.log('Promise resolved')) is moved from the Microtask Queue to the Call Stack and executed.

Important: The Promise callback waits until the current synchronous code (including console.log('End')) has finished executing.

Microtasks vs. Macrotasks: A Comparison

Chaining Microtasks

Microtasks can be chained together. If a microtask adds another microtask to the queue, the new microtask will be executed immediately after the first one completes.

console.log('Start');

Promise.resolve().then(() => {
  console.log('First then');
  return Promise.resolve(); // Returns a new Promise
}).then(() => {
  console.log('Second then');
}).then(() => {
  console.log('Third then');
});

console.log('End');

Output:

Start
End
First then
Second then
Third then

Explanation:

Each .then() callback is added to the Microtask Queue. They are executed sequentially, one after the other, because each one adds the next to the queue.

Potential Issues: Starvation

If you have a long chain of microtasks, it can potentially starve the macrotask queue. This means that UI updates or other important tasks might be delayed. Be mindful of the complexity of your microtask chains. Avoid doing heavy computations directly within microtasks. Consider breaking down large tasks into smaller chunks.

queueMicrotask()

The queueMicrotask() function allows you to explicitly add a task to the Microtask Queue. It's less commonly used than Promises, but can be helpful in specific scenarios.

console.log('Start');

queueMicrotask(() => {
  console.log('Microtask from queueMicrotask');
});

console.log('End');

Output:

Start
End
Microtask from queueMicrotask

Conclusion

Microtasks are a fundamental part of JavaScript's asynchronous behavior. Understanding how they interact with the Event Loop and macrotasks is crucial for writing predictable and efficient asynchronous code. By prioritizing microtasks, JavaScript ensures that important asynchronous operations are handled promptly without blocking the main thread. Remember to be mindful of potential starvation issues when dealing with long chains of microtasks.