WeAreDevelopers LIVE • Sep 18, 2024

The Eventloop in JavaScript - How does it work?

Christian Woerz

Why do recursive promises sometimes freeze your JavaScript application? The culprit is often microtask starvation. Master the event loop to debug complex race conditions and write non-blocking code.

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#1 about 7 min

Differences between JavaScript engines and runtimes

How engines implement the ECMAScript standard while runtimes provide additional APIs like timers and network requests.

#2 about 3 min

Why single-threaded JavaScript needs an event loop

Offloading execution to asynchronous tasks prevents slow, synchronous operations from blocking the single-threaded user interface.

#3 about 1 min

Understanding the role of the call stack

The call stack sequentially tracks and executes all currently running synchronous code.

#4 about 2 min

How the macro task queue handles callbacks

Runtimes utilize a first-in-first-out queue to store and execute completed callbacks from web APIs.

#5 about 2 min

Prioritizing promises with the microtask queue

Resolved promises and async operations enter a high-priority queue that precedes standard callback execution.

#6 about 3 min

Orchestrating asynchronous timers with runtime APIs

External runtime APIs process asynchronous operations before pushing callback results into the appropriate task queues.

#7 about 3 min

Event loop orchestration and execution priority

The loop continually checks the call stack before pulling prioritized tasks from the micro and macro queues.

#8 about 4 min

Visualizing JavaScript execution order with live code

Live coding demonstrates how synchronous code evaluates before asynchronous timer callbacks can enter the macro queue.

#9 about 2 min

Evaluating microtask priority over macro tasks

Immediately resolved promises bypass timer callbacks due to the strict priority given to the microtask queue.

#10 about 3 min

Handling long-running asynchronous HTTP network requests

Network requests delay entry into the task queue until completion, fundamentally altering the expected execution order.

#11 about 5 min

Starving the macro task queue with continuous microtasks

Recursively generating new microtasks blocks the event loop from ever reaching underlying macro task callbacks.

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