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

The JavaScript language uses several built-in symbols to make different features work, for example, the 

 described in an earlier lesson in this module. Such symbols are referred to as "well-known symbols" by the ECMAScript specification.

While the complete list of well-known symbols in the JavaScript language can be found in the 

, this lesson describes some of the well-known symbols.

As explained in an earlier lesson in this module, 

 represents a symbol property that is used by the object to primitive conversion process in JavaScript. Its value is a function that is passed a 

 of the primitive value to represent the object being converted into a primitive value.  The return value is used as the primitive value of the object.

The following code example hooks into the object to the primitive conversion process of the 

 object and returns a different representation of the object based on the value of the 

You can see the output of the above code example in this Replit:

 method isn't very useful for user-defined objects.

console.log({}.toString()); // [object Object]

For some built-in objects, the default implementation of the 

 method is also not useful. As a result, many objects override the default 

const arr = [1, 2, 3];
// overridden implementation
console.log(arr.toString()); // "1,2,3"

// default implementation from Object.prototype
console.log(Object.prototype.toString.call(arr)); // [object Array]

---
title: Well-known symbols
description: Explanation of what well-known symbols are in JavaScript
isPublicLesson: false
useMdx: true
code: https://gitlab.com/fullstackio/books/newline-course-apps/advanced-javascript-unleashed-app/-/tree/main/module_08/lesson_08.05
---

The JavaScript language uses several built-in symbols to make different features work, for example, the `Symbol.toPrimitive` described in an earlier lesson in this module. Such symbols are referred to as "well-known symbols" by the ECMAScript specification.

While the complete list of well-known symbols in the JavaScript language can be found in the [ECMAScript specification](https://tc39.es/ecma262/#sec-well-known-symbols), this lesson describes some of the well-known symbols.

## Symbol.toPrimitive

As explained in an earlier lesson in this module, `Symbol.toPrimitive` represents a symbol property that is used by the object to primitive conversion process in JavaScript. Its value is a function that is passed a _hint_ or the _preferred type_ of the primitive value to represent the object being converted into a primitive value.  The return value is used as the primitive value of the object.

The following code example hooks into the object to the primitive conversion process of the `movie` object and returns a different representation of the object based on the value of the `hint` argument.

```js
const movie = {
  name: "Jurassic Park",
  releaseDate: "09,June,1993",

  [Symbol.toPrimitive](hint) {
    if (hint === "number") {
      return new Date(this.releaseDate).getTime();
    } else {
      return this.name;
    }
  }
};

console.log(Number(movie));
console.log(String(movie));
```

You can see the output of the above code example in this Replit:

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

## Symbol.toStringTag

The default implementation of the `Object.prototype.toString` method isn't very useful for user-defined objects.

```js
console.log({}.toString()); // [object Object]
```