Asynchronous programming in javascript

Here’s an example of a beginner’s guide to asynchronous programming in JavaScript, including code samples for various asynchronous patterns and an explanation of the event loop.

Understanding Asynchronous Programming in JavaScript: Beginner’s Guide to the Event Loop

JavaScript handles operations using synchronous and asynchronous approaches. The key to understanding asynchronous behavior is the event loop.

Synchronous and Asynchronous Code

Synchronous Code executes line by line, blocking subsequent lines until the current line finishes.

Example: Synchronous Code console.log("First");
console.log("Second"); Output: First
Second

Asynchronous Code allows some operations to run in the background and complete later, enabling other code to continue executing.

Example: Asynchronous Code with setTimeout() console.log("First"); setTimeout(() => { console.log("Second");
}, 0); console.log("Third"); Output: First
Third
Second 

Asynchronous Patterns in JavaScript

1. Callbacks

Callbacks are functions passed as arguments to other functions, executed after the first function completes.

Example: Callbacks

console.log(“Start”);

function asyncTask(callback) {
setTimeout(() => {
console.log(“Async task completed”);
callback();
}, 2000);
}

asyncTask(() => {
console.log(“Task finished”);
});

console.log(“End”);

Output:

Start
End
Async task completed
Task finished

1. Promises

Promises represent a value that will be available in the future and can be used to handle asynchronous results.

Example: Promises

console.log(“Start”);

const asyncTask = new Promise((resolve) => {
setTimeout(() => {
console.log(“Async task completed”);
resolve();
}, 2000);
});

asyncTask.then(() => {
console.log(“Task finished”);
});

console.log(“End”);

Output:

Start
End
Async task completed
Task finished

1. Async/Await

Async/await simplifies working with promises by allowing asynchronous code to be written in a more synchronous style.

Example: Async/Await

console.log(“Start”);

async function asyncTask() {
await new Promise((resolve) => {
setTimeout(() => {
console.log(“Async task completed”);
resolve();
}, 2000);
});

console.log(“Task finished”);
}

asyncTask();

console.log(“End”);

Output:

Start
End
Async task completed
Task finished

The Event Loop

The Event Loop manages how JavaScript handles asynchronous operations, distinguishing between microtasks and macrotasks.

Microtasks (e.g., promise callbacks) are executed before the next rendering.

Macrotasks (e.g., setTimeout) are executed after the current stack and all microtasks are completed.

Example: Event Loop with Microtasks and Macrotasks

console.log(“Start”);

setTimeout(() => {
console.log(“Macrotask: setTimeout”);
}, 0);

Promise.resolve().then(() => {
console.log(“Microtask: Promise”);
});

console.log(“End”);

Output:

Start
End
Microtask: Promise
Macrotask: setTimeout

This guide provides a foundation for understanding how JavaScript handles asynchronous operations and the event loop, helping you write efficient, non-blocking code.