Micro-tasks within an event loop (Summary)

Micro-tasks within an event loop: A micro-task is said to be a function which is executed after the function or program which created it exits and only if the JavaScript execution stack is empty, but before returning control to the event loop being used by the user agent to drive the script’s execution environment. A Micro-task is also capable of en-queuing other micro-tasks.

Micro-tasks are often scheduled for things that are required to be completed immediately after the execution of the current script. On completion of one macro-task, the event loop moves on to the micro-task queue. The event loop does not move to the next task outside of the micro-task queue until the all the tasks inside the micro-task queue are completed. This implies that the micro-task queue has a higher priority.

Once all the tasks inside the micro-task queue are finished, only then does the event loop shifts back to the macro-task queue. The primary reason for prioritizing the micro-task queue is to improve the user experience. The micro-task queue is processed after callbacks given that any other JavaScript is not under mid-execution. Micro-tasks include mutation observer callbacks as well as promise callbacks.

In such a case wherein new micro-tasks are being added to the queue, these additional micro-tasks are added at the end of the micro-queue and these are also processed. This is because the event loop will keeps on calling micro-tasks until there are no more micro-tasks left in the queue, even if new tasks keep getting added. Another important reason for using micro-tasks is to ensure consistent ordering of tasks as well as simultaneously reducing the risk of delays caused by users.

Syntax: Adding micro-tasks:

queueMicrotask(() => {
    // Code to be run inside the micro-task 
});

The micro-task function itself takes no parameters, and does not return a value.

Examples: process.nextTick, Promises, queueMicrotask, MutationObserver