event-loop.md

Event Loop. 🔁 😵

Home / Event Loop? 🔁 😵

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Some Concepts or go directly to Event Loops

JavaScript has a concurrency model based on an event loop, which is responsible for executing the code, collecting and processing events, and executing queued sub-tasks. This model is quite different from models in other languages like C and Java.

Single-Threaded means that JS can only run one instruction at a time, even if your CPU has multiple cores and available threads.

Some Runtime Concepts:

➡ Stack 🍔
➡ Heap ☁️
➡ Queue 🍞

Stack 🍔

Heap ☁️

Queue 🍞

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Event Loops 😵

Event Loop is essentially an infinite loop which keeps on checking if something exists in the queue. Its most basic implementation can be imagined as:

while (queue.waitForMessage()) {
  queue.processNextMessage();
}

Here is an example to understand better:

const first = () => console.log("First");
const second = () => setTimeout(() => console.log("Second"), 1000);
const third = () => console.log("Third");

first();
second();
third();

// Output
// JavaScript will first run the synchronous code then it will look for messages in the queue.

First;
Third;
Second;

See the above code here

Zero delays

Now, if we set the setTimeout with a delay of 0 (zero) milliseconds doesn't execute the callback function after the given interval.

const second = () => setTimeout(() => console.log("Second"), 0);

The output will remain the same! 🤯

Non-blocking

A property of the event loop model is that JavaScript, unlike a lot of other languages, never blocks. Handling I/O is typically performed via events and callbacks, so when the application is waiting for a database query to return or an XHR/API request to return, it can still process other things like user input.

Lets recapitulate 📝:

For each round of Event Loop JavaScript will:

➡ Run synchronous code.  
➡ Run [Promise](/promises.md) or microtask callbacks.  
➡ Run asynchronous code.

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See Also:

⚫ Promises 🤝 and Async/Await 🤯 Read ▶
⚫ Datatypes in JavaScript. 😙 Read ▶