JavaScript is a single-threaded programming language, which means that it can only execute one task at a time. This is because JavaScript uses a single call stack, which is a data structure that keeps track of the currently executing functions.

However, even though JavaScript is single-threaded, it is still capable of executing asynchronous code. This is because JavaScript uses an event loop to handle asynchronous operations.

If you're a JavaScript developer, you've probably heard of the event loop. The event loop is a fundamental concept in JavaScript that allows you to write asynchronous code that runs in the background without blocking the main thread. In this article, we will explain how the event loop works and how to take advantage of its characteristics to write better asynchronous JavaScript code.

Understanding the Event Loop

The event loop is a mechanism that allows JavaScript to handle asynchronous operations. It works by continuously checking a queue for events that need to be processed. When an event is found in the queue, the event loop takes it out and processes it, allowing the associated code to run asynchronously.

The event loop has three main components: the **call stack** , the **task queue** and **web api**. The call stack is a data structure that keeps track of the currently executing functions. The task queue is a list of tasks that need to be processed, such as a click event or a network response.

When a function is called, it is added to the call stack. If the function makes an asynchronous call, such as a network request or a setTimeout, it is removed from the call stack and added to the task queue. The event loop continuously checks the task queue and processes any tasks that are found.

Using Web APIs and C++ APIs

JavaScript also provides a set of Web APIs that can be used to handle asynchronous operations. These APIs include the Fetch API, which can be used to make network requests, and the Web Storage API, which can be used to store data in the browser's local storage.

Web APIs are designed to be non-blocking and asynchronous, which means that they can run in the background without blocking the main thread. This makes them ideal for handling long-running operations such as network requests or file uploads.

In addition to Web APIs, JavaScript can also interface with C++ APIs through the use of WebAssembly. WebAssembly is a binary format that can be used to run high-performance code in the browser, including C++ code.

By using C++ APIs, developers can take advantage of the high performance and low-level access provided by C++, while still writing JavaScript code for the user interface. This can be especially useful for handling complex algorithms or simulations that require high-performance computation.

Prioritizing Tasks with Microtasks and Macrotasks

JavaScript has a separate microtask queue that is used to handle certain types of tasks. Microtasks are tasks that need to be processed immediately after the current task has completed, before any other tasks in the task queue are processed. Examples of microtasks include promise callbacks, mutation observers, and animation callbacks.

When a microtask is added to the microtask queue, it is processed immediately after the current task has completed, before any other tasks in the task queue are processed. This means that microtasks are prioritized over other tasks in the task queue, and are always processed before the next rendering cycle.

In addition to the microtask queue, JavaScript has a separate macrotask queue that is used to handle certain types of tasks. Macrotasks are tasks that need to be processed after the current task has completed, and after any microtasks in the microtask queue have been processed. Examples of macrotasks include setTimeout callbacks, network requests, and UI rendering.

When a macrotask is added to the macrotask queue, it is processed after the current task has completed, and after any microtasks in the microtask queue have been processed. This means that macrotasks are processed in a FIFO (first-in, first-out) order, and are always processed after any pending microtasks.

How to Take Advantage of the Event Loop

To take advantage of the event loop, it is important to write code that is non-blocking and does not block the main thread. This can be achieved by using asynchronous functions, callbacks, promises, and async/await.

Using asynchronous functions allows code to run in the background without blocking the main thread. Callbacks, promises, and async/await provide a way to handle asynchronous operations and provide a more elegant way to handle errors.

In addition to using asynchronous functions, it is important to prioritize tasks using microtasks and macrotasks. By using microtasks for high-priority tasks and macrotasks for low-priority tasks, you can ensure that your code is processed in the correct order and that the user interface remains responsive.

Best Practices for Asynchronous JavaScript

To write effective asynchronous JavaScript code, it is important to follow some best practices. Here are a few tips to get you started:

Use asynchronous functions whenever possible. Asynchronous functions allow code to run in the background without blocking the main thread, which can improve the performance of your application.

Use promises or async/await to handle asynchronous operations. Promises and async/await provide a more elegant way to handle asynchronous operations and can simplify your code.

Prioritize tasks using microtasks and macrotasks. By using microtasks for high-priority tasks and macrotasks for low-priority tasks, you can ensure that your code is processed in the correct order and that the user interface remains responsive.

Use Web APIs for long-running operations. Web APIs are designed to be non-blocking and asynchronous, which makes them ideal for handling long-running operations such as network requests or file uploads.

Use C++ APIs for high-performance computation. By using C++ APIs through WebAssembly, you can take advantage of the high performance and low-level access provided by C++, while still writing JavaScript code for the user interface.

Conclusion

The event loop is a fundamental concept in JavaScript that allows developers to write asynchronous code that runs in the background without blocking the main thread. By understanding how the event loop works and how to take advantage of its characteristics, developers can write better asynchronous JavaScript code that is non-blocking and does not block the main thread.

By using microtasks and macrotasks to prioritize tasks, developers can ensure that their code is processed in the correct order and that the user interface remains responsive. By using Web APIs for long-running operations and C++ APIs for high-performance computation, developers can take advantage of the features provided by these technologies while still writing JavaScript code for the user interface.

By following these best practices and staying up-to-date with the latest developments in asynchronous JavaScript, developers can write code that is performant, scalable, and maintainable.

Interview Question

One of the most commonly asked interview questions about this subject is to describe the following code and tell the interviewer what will be printed into the console:

for(var i = 0; i < 10; i++){
  setTimeout(() => {
    console.log(i)
  }, 0)  
}

Many people fails at this question guessing the answer is: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.

What is happening in this code

The JavaScript code above creates a loop that iterates from 0 to 9 (inclusive) using a for loop. Within the loop, a setTimeout function is called, which takes two arguments: a callback function and a delay time. In this case, the delay time is set to 0 milliseconds, which means that the callback function will be executed immediately after the current loop iteration.

The callback function simply logs the value of the loop variable i to the console using the console.log() method.

However, there is a caveat to this code: because the setTimeout function is asynchronous, the callback function will not be executed immediately, but will instead be added to the event queue and executed once the current execution stack is empty. This means that by the time the callback function is actually executed, the value of i may have changed, because the loop has already finished iterating. As a result, the code will log the value of i as 10 ten times, rather than logging the values from 0 to 9.

How to make it logging from 0 to 9

1. Passing the value of i as argument to setTimout callback

for(var i = 0; i < 10; i++){
  setTimeout((j) => {
    console.log(j)
  }, 0, i)  
}

2. Creating a closure by using a IIFE (Immediately Invoked Function Expressions)

for(var i = 0; i < 10; i++){
  ((j) => {
    setTimeout(() => {
      console.log(j)
    }, 0)  
  })(i)
}

3. Probably the easiest solution is to use es6 feature "let"

for(let i = 0; i < 10; i++){
  setTimeout(() => {
    console.log(i)
  }, 0)  
}

The scope of i is a block scope, and it lives inside brackets {}. It means that in every loop the value of i will be stored in the upper scope of the setTimeout callback