Course Overview

Overview of history of the JavaScript language

History of JavaScript

Overview of the relationship between JavaScript and Java

JavaScript vs Java

Overview of how JavaScript is evolved as a programming language

How is it evolved?

Overview of how we can track upcoming features in the JavaScript language

How to track upcoming features?

Overview of how the ECMAScript is versioned

How is ECMAScript versioned?

Explanation of whether JavaScript is interpreted or a compiled language

Is it interpreted or compiled?

Explanation of how JavaScript is executed

How is it executed?

Execution Context

Explanation of what is a callstack and how it is used in JavaScript

Callstack

Overview of memory management in JavaScript

Memory

What is JavaScript

What is hoisting?

Explanation of common misconception about hoisting in JavaScript

Misconception about hoisting

Hoisting

What is scope?

Explanation of global scope in JavaScript

Global scope

Explanation of function scope in JavaScript

Function scope

Block scope

Explanation of module scope in JavaScript

Module scope

Scope chain

Scope

What is coercion?

Diving into inner details of  coercion in JavaScript

Deeper look at coercion

Explanation of what abstract operations are and common types of abstract operations in JavaScript

Abstract operations

Explanation of how abstract or loose equality works in JavaScript

Abstract equality operator

Explanation of how the addition operator works in JavaScript

Addition operator

Explanation of how relational operators work in JavaScript

Relational operators

Coercion exercise for testing and practicing the knowledge learned in this module

Coercion exercise

Coercion

What is a closure?

Explanation of a common misconception about closures in JavaScript

Misconception about closures

Explanation of the problem of closures inside loops and how to fix it

Closures inside loops

Explanation of closures can be used to hide data

Data hiding using closures

Closures

What is prototypal inheritance?

Prototype chain

__proto__ property

Explanation of how we can create objects with custom prototypes in JavaScript

Custom prototypes

Explanation of how prototype chain is set with ES6 classes

ES6 classes and prototypes

Prototypes

Explanation of this keyword in JavaScript

What is this?

Explanation of how "this" works inside arrow functions

Arrow functions and "this"

Explanation of how value of "this" can be set explicitly

Explicitly binding "this"

globalThis

Summarizing "this"

'this' keyword

What is a symbol?

Explanation of why symbols were added in JavaScript

Why symbols?

Symbols and privacy

Explanation of global symbols in JavaScript

Global symbols

Explanation of what well-known symbols are in JavaScript

Well-known symbols

Symbol

Overview of asynchronous behavior of JavaScript

Overview

Overview of how callbacks help us write asynchronous code in JavaScript

Callbacks

Event loop

Promise

Explanation of promise chaining in JavaScript

Promise chaining

Explanation of some static methods of promises and their use cases

Use cases for static Promise methods

Explanation of async-await syntax in JavaScript

async await

Explanation of microtasks, how they are processed, and how they are linked to promises

Microtasks

Explanation of common anti-patterns to avoid when using promises in JavaScript

Promise anti-patterns

Asynchronous JavaScript

Explanation of what is an iterator in JavaScript

Iterators

Explanation of what is a generator in JavaScript

Generators

Explanation of how we can create asynchronous iterators in JavaScript

Asynchronous iterators

Explanation of how we can create asynchronous generators in JavaScript

Asynchronous generators

Iterators and Generators

Overview of what is debugging and different ways to debug JavaScript

Debugging

Overview of using the "debugger" statement to debug JavaScript

Debugger statement

Overview of using the breakpoints in browser developers' tools to debug JavaScript

Breakpoints in browser

Overview of using the VS Code debugger to debug JavaScript

VS Code debugger

Debugging JavaScript

Final thoughts and advice regarding how to continue the exciting journey of learning JavaScript

Conclusion

Wrap up

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Become a confident JavaScript pro by unraveling the most challenging aspects of the language. With easy-to-understand explanations and code examples, you’ll gain a deeper and solid understanding of JavaScript's most complex features.

Advanced JavaScript Unleashed is for beginner and intermediate developers alike, who want to elevate their JavaScript skills and become senior engineers by gaining a deeper understanding of the core topics of the language.

Advanced JavaScript Unleashed

Using callback functions to handle asynchronous code has been the traditional way of writing asynchronous code in JavaScript. A callback function is a function that is passed to another function as an argument and is intended to be invoked after some asynchronous operation. The function that receives the callback function as an argument typically invokes the callback function with the result of the asynchronous operation or the error if the asynchronous operation fails.

Operations like HTTP requests are asynchronous, but they aren't handled by JavaScript. The code we write initiates the asynchronous operation; the actual asynchronous operation is handled by the browser in the case of client-side JavaScript, background threads, or the operating system itself in the case of the NodeJS runtime.

In simple words, asynchronous operations take place in the background (outside of JavaScript land), and in the meantime, other things can execute on the main thread (in JavaScript land).

When the asynchronous operation is completed, our JavaScript code is notified, leading to the execution of the callback function that we provided at the time of initiating the asynchronous operation.

This is how JavaScript gets around its limitation of a single thread. The asynchronous operations are actually handled by the runtime (browser, NodeJS, etc.). In the meantime, JavaScript can do other things.

The following code example shows the use of a callback function by sending an HTTP request to a fake REST API:

You can run the code above in the Replit below:

 method handles the result of the HTTP request.

Different DOM events, for example, the click event, are also handled asynchronously using the callback functions. A callback is registered as an event handler and is invoked later whenever the event is triggered.

const submitBtn = document.getElementById("submit");

submitBtn.addEventListener("click", function (event) {
  // code to handle the click event
});

Similarly, different timer functions like 

 are also provided with a callback function that is intended to be invoked after the specified amount of time has elapsed.

setTimeout(function () {
  console.log("logged after 2 seconds");
}, 2000);

---
title: Callbacks
description: Overview of how callbacks help us write asynchronous code in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_09/lesson_09.02
---

Using callback functions to handle asynchronous code has been the traditional way of writing asynchronous code in JavaScript. A callback function is a function that is passed to another function as an argument and is intended to be invoked after some asynchronous operation. The function that receives the callback function as an argument typically invokes the callback function with the result of the asynchronous operation or the error if the asynchronous operation fails.

Operations like HTTP requests are asynchronous, but they aren't handled by JavaScript. The code we write initiates the asynchronous operation; the actual asynchronous operation is handled by the browser in the case of client-side JavaScript, background threads, or the operating system itself in the case of the NodeJS runtime.

In simple words, asynchronous operations take place in the background (outside of JavaScript land), and in the meantime, other things can execute on the main thread (in JavaScript land).

When the asynchronous operation is completed, our JavaScript code is notified, leading to the execution of the callback function that we provided at the time of initiating the asynchronous operation.

This is how JavaScript gets around its limitation of a single thread. The asynchronous operations are actually handled by the runtime (browser, NodeJS, etc.). In the meantime, JavaScript can do other things.

The following code example shows the use of a callback function by sending an HTTP request to a fake REST API:

```js
function fetchUser(url) {
  const xhr = new XMLHttpRequest();

  xhr.addEventListener("load", function () {
    // check if the operation is complete
    if (xhr.readyState === 4) {
      if (xhr.status === 200) {
        // Request succeeded
        const data = JSON.parse(xhr.responseText);
        console.log(data);
      } else {
        // Request failed
        const error = new Error("Failed to fetch todo");
        console.log(error);
      }
    }
  });

  xhr.open("GET", url);
  xhr.send();
}

fetchUser("https://jsonplaceholder.typicode.com/todos/1");
```

You can run the code above in the Replit below:

<ReplitEmbed src="https://replit.com/@newlineauthors/callbacks-example1" />

A callback function passed to the `addEventListener` method handles the result of the HTTP request.

Different DOM events, for example, the click event, are also handled asynchronously using the callback functions. A callback is registered as an event handler and is invoked later whenever the event is triggered.

```js
const submitBtn = document.getElementById("submit");

submitBtn.addEventListener("click", function (event) {
  // code to handle the click event
});
```

Similarly, different timer functions like `setTimeout` are also provided with a callback function that is intended to be invoked after the specified amount of time has elapsed.

```js
setTimeout(function () {
  console.log("logged after 2 seconds");
}, 2000);
```

You can run the code above in the Replit below: